Rooting basically gives your "Administration" access to your phone. You will be able to browse system files with a file browser and also be able to remove system files. You gain CPU overclocking, which is very nice for these little phones. WiFi tethering is another addition that I have used many times. You'll also be able to install a custom ROM, and there are some very good custom ROMs out there for the Optimus, on VD and VH baseband.
EDIT: Also, with most custom ROMs you'll gain about 50-80mbs of internal storage. Which I believe is a must for the Optimus which has very little internal memory.