There are a lot of things you will be able to do once your phone is rooted, but I think the most direct benefit is being able to use wi-fi tethering without paying Sprint $30.00/mo. Two other key apps are Titanium Backup (backs up all your apps)
and Root Manager (allows you to delete unwanted system apps and bloatware). Plus there are a lot of apps in the Market that you need root access to run.
Once the devs get some custom ROMs out there you will be able to experiment and see what you like. I'm sure the typical menu will be ROMs that focus on speed, some that focus on battery life, and some that focus on eye candy. They can change the radio settings to get better performance out of your phone while saving battery life, or they can streamline the functions of the phone so that apps open faster and smoother.
If you want to get an idea of what kind of custom ROMs will be coming, go to the forums for phones that have been out for awhile like the
HTC EVO 4G or the
HTC Touch Pro 2 you will see what's cooking.
The most important thing you need to do before you start flashing ROMs on your phone is READ! Read everything you can find about how all this is done and how to fix it if you screw it up. Go join
xda-developers too - they are another excellent source of information on how to modify smartphones.
I almost bricked my TP2 because I thought I could just follow the instructions on the screen and everything would happen like it said it would. That is almost never the case. There are so many variables that could cause what you are doing to fail and you need to be aware of all of them before you dive in.
Enjoy!