SCX adds more platforms, improves models/graphics, and tweaks gameplay as well. The benefit to the extra platforms is simply to add to the variety of possible scenarios.
SCU is basically a companion to SCX that makes many of the other submarines playable, just one at a time. It doesn't do them all at once, that's the limitation. It essentially creates a fourth platform that can be changed by using scPlayableSubChanger.exe. You change it and load the game to create/play your scenarios. You can also use the new sub in other missions that don't designate a specific platform. To change it up again, you have to exit the game and run scPlayableSubChanger.exe again.
By the way, my two sentences about SCX don't really do it justice.
As you get more and more into the game you'll realize all the work that went into that, truly one of the great community mods for any game I've played.
Hopefully this helps.
-UB