Why do people put 70mm TB's on 96-98 GT's

[91LX2Bfast]

I have one on my car because I picked it up used for fairly cheap, and I knew I would be adding bigger power sometime in the future (as in I just purchased a nitrous kit.) So far I am happy with the combo, as my car runs just as hard a 96-98 GT auto's with bolt on's and yet my car is 400+lbs heavier. So I figure why try to mess with a good thing if my car is running well.
I guess I could go to the track and make some runs with the 70mm, and then make some runs with the 65mm, and see if there is any difference, I figure and difference will be impossible to tell w/o a chassis dyno. So for now, it stays on. Plus you get better throttle response from the stiffer return spring. I wouldn't recogmend to put one on a bolt on engine, there are far to many better mods to do first, but if you can get one on the cheap like I did, then why not.