The iPad has a lot more components than say, a Macbook, because these smaller processors are specialized to handle normally CPU-heavy tasks, such as playing videos. Hardware specialization is especially important for mobile devices like these because for such tasks we don't want to be running the CPU at 100% capacity because it will drain the power too quickly.
Side note, the image processing DSP handles touch screen events (and the image processor on the main chip handles the camera).
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To add onto @stephyeung, hardware specialization allows for minimal power usage. This is crucial for iPad like devices to promote battery life. Additionally, preprocessing also allows for saving on bandwidth.
As pointed out in the second exam, there are APIs available that decode video and audio using the chips outlined above, instead of having to do it in software. I dug around for a couple minutes and found https://developer.apple.com/library/mac/#documentation/AudioVideo/Conceptual/AVFoundationPG/Articles/00_Introduction.html. Apparently that does use special chips on iOS devices.
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As pointed out in class, the image processing DSP on the left is used to answer screen touch. Since it is dedicated to do this task, iPad can provide very smooth experience to users.