Mostly that you can remove stock apps you don't want and you can test improvements in restricted areas (for instance /system and /data) easier.
Also some programs need root access to run; especially those fiddling with settings / performance
One downside to rooting: it gives you the opportunity to break your configuration (not literally). If you remove/alter some files, your system can get unstable or unbootable. Usually you can restore that by reflashing a stock rom or just replacing the edited file by the original
T-Mobile is a little bit late for this phone + they don't have same GT-i9000 model. (they will have model without front facing camera).
I think fring/skype/facebook video and other IM messangers will become very popular in future. So people will more and more talk over Internet then on GSM. Because in some countries there is 3G flatrate package and also 4g (802.16e WiMAX) will bring us to new era of mobile industry.
you can download apps from nexus one section or get your hands dirty on android themes and apps section, and even install cracked android apps and games ... do you know what root means .. like a tree it has root, you can add urea and fertilizers to it to make it grow better and bigger .. same case with this ...