I am an AMX Guy and here are some of the things I do with two-way.
Easily give feedback as to what source is on in a room. -very nice when there is multiple sources of the same thing,ie.4 cable boxes.
Many devices provide status of the product. -volume -lamp life -power state -input mode -You can use this information to update the touch panel based off of the information you need.
I love to give the feed back for alarms and lights on touch panels. Easily see what lights are on/off and what window and doors are open.
I will also do things with automation like -If johny disarms the alarm -do a bunch of things -change his touch panel to a certain page
It is neet to have things triggered by an event but if the touch panel does not know about it, the end user may forget what source is on and accidently change it. Again the several cable boxes point.
These are just a few items. With the two way world, you get to tell the customer "YES" way more offten when they ask "Can my system do that"
Don't read my answer. Someone else will go into great detail as to why I am wrong rather than answer the original question...