The main purpose for the Firewire port ended up networking GT3.
A few other games used it, too, but not many.
The port was dropped in the redesigned console and that is one of the reasons why we have to use Ethernet networking protocols with GT4, so it can go on-line. Firewire ports can't do that, as far as I know, at least not in a "standard" way.
Cheers,
MasterGT