What would be gained by rooting a phone?
What would be gained by rooting a G2?
In its purest form, rooting merely gives you super user rights. In effect, you get admin rights to your phone's OS the same as you would your desktop/laptop. Thats it.
What you can do with root, now, thats a different story. For starters, you could remove the carrier/manufacturer bloatware. My Thunderbolt, for example, has 8 pre-installed, unremovable apps from Verizon. Without root, its just junk taking up space. With root, I can remove them, clearing up storage space and potentially RAM. You can remove the manufacturer's craptastic skin, Sense, Blur, TouchWiz, Espresso, etc. Without root, you can only install another launcher to cover it up, with varying levels of success depending on the skin.
What most people do with root, though, is flash custom Android builds to their phone. These builds vary widely, but can simply be 'tweaked' versions of the stock build or they can be complete rebuilds compiled from Android source. Most of the builds for the Thunderbolt are tweaked stock builds at the moment, but all the tools are in place for full AOSP builds. Just a question of time really.
In the case of a TMO G2, its been out for a while, so you have wide options with builds you can flash to the phone.
Be advised, that this does void your warranty. If you brick your phone to an unrecoverable state, you're going to pay full price for a replacement. Some phones carry higher levels of risk with rooting, especially when the phone is 'young'. Others, its a walk in the park. The G2 should be pretty easy to root now.