incorrect. that's exactly what you can do with a rooted droid. you can download different ROMs (think interface), delete bloatware ie all of the garbage apps that come stock, run free wireless tethering, change boot screens, run apps off of your memory card as opposed to having to store on the phone's internal memory, as well as over/under clocking your processor, etc. rooting a droid basically gives you full control of the device to do with as you please.
however, if you don't know what you're doing, and/or don't know where to look online, then rooting is pointless.
And OP - you cannot take an AT&T (gsm) phone and root it to make it work for verizon (cdma). the technology is completely different. basically, you've got a device that you can use for wifi, and as an mp3 player, but it will never work on your current service provider. sell it on ebay.