an excellent question. A full version of my answer can be found in my latest book listed here, The design Students Journey: understanding how designers think. Here is a simplified version of one part, in my view a key part, of my answer.

although we have added cad to our arsenal of tools as designers, none of these systems so far has provided the directness and rapidity and flexibility of human sketching. We now recognise that designing is less a matter of problem solving as it is a conversation between design and the problem situation. In short, a designer externalises some possible parts of a solution and then looks at the drawing , learns about their situation, modifies the drawing and so on. schon talked wonderfully about the reflective practitioner in May professions but in design his ideas are seminal. Designers converse with their drawings or sketches.

today students are arriving at design school, certainly in architects and similar subjects, having used computers through their childhood and perhaps not learned to record the world by sketching. This then leaves them relatively poor at this central design skill. Teaching them to learn to sketch by looking and seeing and thus having a conversation with their drawings is thus one of the most important things we can do to develop their whole design process.

I should have mentioned the important addition of the digital camera now in all our phones. This means that to record something or place that might be of interest to a designer and may be useful in a future design situation, all the student has to do is press the shutter button. Previously we would have had to sketch. Now to do this requires us to pass the image through the eye-brain-hand system and thus process it and analysize it. No analysis takes place when using your mobile phone. So yet another importnat skill needed by the design student is not being learnt outside the design school studio in the real world. Even more dangerous for the future designer who simply does not have the practised skills of brain-hand-eye coordination need to sketch. Since sketchign is a central and vital part of designing this is a serious deficiency and hte design school must remedy it by formally teaching sketching.

In the Strugatsky science fiction story, an astronaut who fell into another civilization was tested for the ability to display an object using a drawing. Since ancient times, this ability has been an integral part of the formation of spatial thinking. The computer turned this ability into a complete concrete model excluding spatial abstraction. This was reflected in the iconic and stylistic uniformity of modern products.

While I’m not an architectural researcher, I do draw, and use drawing as an integral part of my problem-solving process.

Drawing, sketching, even doodling are not just products; they are evidences of a conversation between the mind and the world expressed as a visual record. Drawing can be the purest form of human ideation. Drawing serves multiple purposes, some of which are outlined by the esteemed colleagues above. Sketching as mimetic record, such as traditional analytical or academic drawing, provides an deep understanding of three-dimensional forms. It enables the observer/recorder to gain a unparalleled grasp of the functioning of the form recorded, reflecting both surface and internal structures. Not just what something looks like, but how it actually works, the underpinning spatial interconnections which make it what it is. This provides a grounding which provides the mind and hand a vocabulary with which to design, a context within which to work, and develops a mechanical skill of expression. It is a foundation upon which many skills and creative disciplines can be built. Sketching/drawing then also serves as the means of pure ideated expression, a negotiation between mind, hand, and eye using the recording medium. These drawings may form as response to photographs or other ocular stimuli, or as unalloyed  phantasia, representing images found only in the individual mind’s eye. The drawing and redrawing of details in a design facilitates their thorough working out and refinement, at a level which makes their translation to any CAD program easier. The medium for such drawing may be real or digital, or mixed, though few architectural CAD programs offer the freedom and fluidity found in simple pencil and paper. The drawing process has many parts; reference photography, thumbnail rough ideas, the working out of details, CAD work, all these elements may inform the mind as it draws. Drawing starts with the eye. The physical process of drawing, the hand-eye labour of sketching, long viewed as an artisanal labour of low aesthetic order,  has itself a highly valuable neurological aspect which enables the brain to access its more creative potential, by triggering a state in which the physical activity of drawing allows the white matter channels in the brain to engage with design problems and offer creative adaptivity, making connections between distant parts of the cerebrum, thereby  producing a richer expression of variant ideas. (Such links are outlined here; Takeuchi, et al: http://jtoomim.org/brain-training/white%20matter%20creativity.pdf) Drawing/sketching is an essential part of any creative design activity.

I agree with all the foregoing and when I taught CAD at university and a college it invariably started with manual drawing. Another reason is in comparing the two systems one appreciates more what one is doing in CAD. Sometimes CAD almost makes it too easy so that the medium informs the design and two dots are joined up or a circle, or part circle formed and so on just because the tools are there. I find I can invariably tell a CAD designed building from a manually drawn designed building. Sometimes the details are better when produced manually, such as weathering to a coping which may be left flat when done in CAD. Of course sometimes it works the other way around, a really cpmolicated detail is easier in CAD. CAD does lead to some of the benefits of BIM. Maybe the solution is a combination of the two as suggeted already.

Why is sketch so significant in the training of spatial intelligence, whether it be for architecture or visual arts….When one uses the hand to draw ideas, it becomes an extension of the mind. The hand and mind work together to express and facilitate mental ideas where changes, additions, and space take form and can be constantly altered to fit the flow of thought. Technological tools cannot duplicate this ability to “feel” one’s creations and how they flow into reality. That is the thrill of being creative and developing a vision. Just like a signature, drawing and sketching allow the person to develop a style which is individualistic and not just one that looks like a computer-generated concept. To sketch allows the mental process a chance to detail an idea into formation without the limitations of technology, and they are limited compared to our natural way of expression. Technology can limit a presentation because of the programing variances. Our minds are not limited except by our own sense of fear and lack of confidence. To sketch is a natural function of all children and adults. We do not want to train this out of people just because other methods are available. We can enhance our creative process by various methods for many solutions to problem solving.

One addition to my comments...remember it is the human mind which has created technological advances. These advances are not meant to replace the creative thinking process, but rather to enhance our ability to express and create more effectively.

Perhaps Charles Garnier would give up the computer today and draw such an architectural detail with a pencil faster