It was simply the top of the line in the T series, which were the latest and greatest of the FD mount. Some might prefer the AE series for their retro charm.
The T90 has the most modern 'canon' feel of the T series, feeling somewhat like a proto-EOS with a more modern style under-finger dial control for adjusting aperature/shutter speed. The T90 development was largely about ergonomics, and the results of that were also used to restart the canon line with the true AF based system with the EOS, the modern canon dslrs still build on this wealth of ergonomics research. The T70 in comparision has a bit of a weird 80s feel to it, but it's perfectly usable once you're used to the buttons instead of dials.
The T60 technically came after the T90, but it was a budget 'retro' camera with more 70s styling, and lacked most of the auto exposure options of the T70 and T90.
If you fancy using the weird FD mount autofocus lenses, I think you need the T80, but other than the auto-focus options, the T80 also works as a perfectly good T70.
Basically, if you want 70s feel, go AE-1 or something else in the AE series or the T60, if you want a camera that is cheap but dependable, and don't care too much about it being 'consistant' with 70s feel or modern feel, go T70. If you want a FD mount camera that feels like a modern dslr, go T90. If you want an autofocus FD mount camera, go T80. I don't think there's really anything that justifies the T50 for today, it's just a (then) budget T series when all the rest of the T series are cheap by today's standards.