I'm no expert, but this is my experience.
The Pic button give you quick access to all the functions for whatever device is displaying your picture (such as a TV or Front-projector), and the Sound button gives you quick access to all the functions for the device that is running your sound.
From my experience, this only really provides much use if you are using a separate sound system, and watching something other than just TV. I have two Harmony remotes; one for my theater, and one for a very simple tv/dvd/vcr setup in another room.
When I'm watching TV on my secondary, more simple setup, the "Pic" and "Sound" buttons don't really serve a purpose, since the TV is providing the source signal (tv tuner), and outputting the sound.
But when I'm, say, watching a DVD in my theater, that is when the "Pic" and "Sound" buttons are valuable (to me at least), as I can access all functions for my front-projector (via Pic), and my Receiver (via Sound).
You can, of course, add functions from ancillary devices to the normal "Activity" button setup, but the Pic and Sound buttons really provide any and all functions which may not fit into your Activity button mapping.
Some people find these buttons quite useless, but I do use them. It would be better for all if these buttons were user-customizable (meaning the ability to assign the button to another function), but unfortunately they are not.
Hope that helps!