Strictly to go fast @ a cheaper price vs the Z. But at the same time, you're paying alot more to get the Supra in the first place. So it'll come out even $$$-wise in the end.
I too considered getting a Supra TT. But for me, it was because it was a fast car different from a Z. I already have a 2+2 Z that I was gonna keep. Why not just get a Supra. But the love of the Z kept me away from it. And I'm getting a TTZ instead.