Besides, what is the point of taxes in a pure fiat money system?
This is simple. It's obviously not to fund the government. So the only meaningful aspect of taxation is the fact that taxes are "progressive". The point is to stimulate constant economic "growth" by preventing anyone from becoming self-sufficient.
I would say that it is just to make seem that fiat money is worth something, and force ppl to work for the government (paying taxes to it).