The income tax was always used to promote certain behaviors and discourage others.
Some examples are deductions for mortgage payments and charitable donations.
The purpose of having rich people pay more tax is that they are able to pay more. This is not to say that there is waste in Washington. There are many ways that the money collected from income tax could be spent better. But a lot of money is spent on special interest programs that are pushed through Congress. I have not seen a good plan from either candidate dealing with this.