It's invaluable, especially if you have different sets of monitoring equipment. It also allows you to put analysis plugins across your out put paths. Can be a little confusing at first, but definitely worth reading up on.
Yes. The Control room is fantastic. It acts (amongst many other things) as a "listen" output. You can, for example, have room correction software and you don't have to worry about the affects and effects of that when exporting audio as it's not on the Master output. Any plugin you have on an insert in the Control Room will never be part of the Master output.
You also can set up the "Listen" functionality which over the years I have found invaluable (especially when you have tracks already soloed - and then need to essentially Solo something whilst they are soloed.
Switching between different Monitor configurations is so simple.
Different metering for monitoring your outputs are also available.
Headphone sends (again - more monitoring) can be easily accessed and controlled from the Control Room.
I only wish there could be more than 4 Headphone sends set up but it's not the end of the world.
Oh, and checking mixes in Mono is a breeze.
There's tons more stuff it can do. Definitely RTFM and feel the power of the Control Room man!
The Control Room Mixer is maybe Nuendo's (and now Cubase's, too) most unique feature. It really makes the DAW behave like a "real" studio, and then some.
... Steinberg somehow managed to destroy the straight-forward "one window" approach the Control Room Mixer initially offered by introducing rather unhandy tabs with Nuendo v6 ff., but the basic idea is still second to none.