Typically, a child learning to read will use a finger to guide his or her eye along the page, but the impulse to do so holds for beginners of all ages.  Any text written in script that is unfamiliar to us tends to be more difficult to "see" and follow (e.g., the East Asian and Altaic languages, which traditionally run from top to bottom, or Classical Greek, which used to be written across the page from left to right and then back from right to left and so on), and a finger placed alongside or underneath the text, by focussing the eye on a single word or phrase at a time, really does help to ease eye strain and improve concentration.  As we become more familiar with a particular script, most of us will gradually fall out of the habit of using a guiding finger because it no longer makes much of a difference.  Children are also discouraged from continuing this practice for too long because it makes them appear "slow"-- like keeping your training wheels after you've already learned to ride a bicycle.  The same goes for sounding the words out as one reads (either aloud or under one's breath)-- this is what we do naturally as children or neophyte readers, and something that also tends to fall by the wayside as we become more practiced at reading.  (Anyone who remembers what fun it was to read aloud, and how beautiful the words sounded, will no doubt feel a pang of regret, and those who like poetry will likely agree with me that poetry is best read aloud, at any age.)  I remember, as a youngster, hearing many times that this practice is only for the semi-literate or the under-educated, so that, as one graduates into full literacy, one should be able to read silently, and that is better because it makes for faster reading (true) and is even more high-class (so not!).  So these two practices, finger-reading and sounding out the words, are (1) "childish" practices that we should grow out of as we mature because they are also  (2) indications of a "lower" (less evolved) level of intelligence and/or education and also, by implication, of lower (less desirable) socio-economic status.

Some one reads quickly some one is slow. It depends on many things. As we grow up we read one word as a pattern. We do not read a word by reading its constituent letters sequentially. There are people who can read a sentence as an item. I heard that there are people who can read a group of sentences as a single object. I think the whole act of reading depends on habit and skill. Therefore use of ones finger while reading will make the process slow.

For students who are struggling to learn to read, many have initial difficulties with tracking of text.  Having a child use their finger to assist with tracking as an initial scaffold of support is common in many of the evidence-based reading interventions.  As the child's skills develop the use of the finger as a scaffold should slowly be faded. 

To follow up on my previous post, I think that, apart from its more obvious application to cognitive science, the study of this has sociological dimensions that should not be overlooked.  For example, how much of the perceived need to wean learners from the use of a guiding finger comes from practical considerations, and how much from the kind of unconscious prejudice that I described earlier?  I would argue a little of both.  Since most readers will fall out of the habit once they become truly proficient-- when they find that they can read faster than a moving finger-- I wonder what purpose it serves to intervene in the natural evolution of their reading skills.

For students who are two and three grade levels below their expected performance on indicators that correlate highly with more complex reading outcomes, the need to intervene can potentially alter the trajectory of their reading growth in a positive way that may reduce dropout rates and improve college readiness.  Something (poor previous instruction, language exposure, cognitive processing deficits, etc.) has caused children identified with severe reading deficits to fall short of expectations based on the "natural evolution of their reading skills."  Hence, the need to intervene to address the issue.  

Thank you for the insights. I was quite confused, since we are taught at some stage in our lifes not to use our fingers - and I get the feeling my own mom told me not to!

To Michael Solis: Thank you for the clarification. I didn't mean to imply that students who are (for whatever reason) at a disadvantage should be left to fend for themselves; every effort should be made to even the odds and help them reach full proficiency.  What I was trying to say was that we should consider the possible influence of cultural and social biases on the behavior of educators as they define their goals and refine their methods for achieving those goals.