Government performs certain essential functions, from education to national defense. It must raise money to do that. Charging everybody the same tax rate might sound simple. But it would actually impose a much harsher burden on the poor, since they end up spending much--if not all--of their incomes on the basic necessities of life, such as food, clothing, and shelter. As one famous 18th century philosopher argued,
“It is not very unreasonable that the rich should contribute to the public expen[s]e, not only in proportion to their revenue, but something more than in that proportion.”
Another rationale for progressive taxation is the fact that random chance has profound effects on everybody’s financial well-being. (A guy named John Rawls once wrote a thing or two about this.) Mandating economic equality--i.e., carrying out a truly socialist agenda--would obviously be wrong. But there are compelling moral and economic arguments for asking the fortunate to pay a little more in taxes, in order to blunt the influence of chance on people’s lives.
Among other things, it’s not clear how long a capitalist society would even survive without at least some redistribution, given the likelihood that--without it--the poor would get poorer and the rich would get richer.