Nice question. Prioritization of traffic based on QoS measures such as delay, jitter and loss can be used to accomplish various objectives. Time sensitive real time voice and multimedia (like gaming and conferencing) services need to be prioritized over best effort applications (email/ internet browsing). 

The basic advantage of QoS based prioritization is to insure efficient use of the network resources such as the bandwidth while serving the needs of the different services requiring different QoS parameters (Real time voice/video tolerate loss but are delay sensitive while internet browsing/email tolerate delay but are loss sensitive).

The disadvantage of QoS prioritization is that it gives no guarantee on the overall user experience. QoS based prioritization along the different components on the data path(s) don't necessarily sum up to provide a total user (quality experience). In other words, QoS prioritization is device centric and we need some measures that are more user centric that can ensure that the call is running smoothly and the download is not taking more than necessary. This is the bigger concept of the Quality of Experience. Thanks. @AlDmour.

Thanks Aldmour. I want to add this contribution to what you said

Since different applications (e.g. telephone, e-mail, surveillance video, etc.) may be using the same IP network, there is a need to control how network resources are shared to fulfil the requirements of each service. One solution is to let network routers and switches operate differently on different kinds of services (voice, data, and video) as traffic passes through the network. By using Quality of Service (QoS), different network applications can co-exist on the same network without consuming each other’s bandwidth.

The term, Quality of Service, refers to a number of technologies such as Differentiated Service Code Point (DSCP), which can identify the type of data in a data packet and so divide the packets into traffic classes that can be prioritized for forwarding. The main benefits of a QoS-aware network include the ability to prioritize traffic to allow critical flows to be served before flows with lesser priority, and greater reliability in a network by controlling the amount of bandwidth an application may use and thus controlling bandwidth competition between applications. Pan–tilt–zoom (PTZ) traffic, which is often regarded as critical and requires low latency, is a typical case where QoS can be used to guarantee fast responses to movement requests. However the prerequisite for the use of QoS within a video network is that all switches, routers and network video products must support QoS.

Thank you, nice addition. This supports the say (I read it some where) that quality sum does not some up to quality. Hence, even if all routers on the path support QoS, no guarantee is available that one priority packet at one router will be prioritized everywhere as some other one might have to be more prioritized. Thanks. @Aldmour

Yes I agree that there is no guarantee that one priority packet at one router will be prioritized everywhere because there may be other higher priority packets at another router.