[a_c_s]

The gist of why you do it is that the carriers develop their ROMs, which are 6 months old by the time the phone is launched in volume, and a lot changes... they typically offer one OS update on a handset if you're lucky... the chefs will cook up ROMs with all kinds of goodies embedded as others have mentioned (wifi, facebook, etc) as well as the newest registry tweaks and mods to make the phones function more quickly and with better battery optimization...once you try custom ROMs you won't believe the difference to be honest...