Starting with the 659, Harmony added a Sound and Picture button. Pressing Sound while in an activity is similar to pressing Device, then selecting the device controlling the Sound for that activity (A/V receiver, TV, etc.) except that the hard and soft buttons can be configured independently of the device itself. e.g. if you're watching TV you can program all of the various sound fields for the A/V receiver or any other settings. Picture does the same thing but only for the Display device (tv or projector). You can use it to set picture modes or display properties.

You press each button once to activate and again to return to the activity (whereas device mode requires 2 button presses each way.

You can put other commands in each one by teaching those commands to the Sound or Display device.

If you use a lot of sound fields or picture modes (still haven't figured out why you need to do that - set it accurately and forget it) they're indispensible. If you haven't used them you probably wouldn't miss them.