One big reason is that people who don't have much money tend to spend what they have. The very rich tend to invest their money, frequently overseas. So by redistributing wealth you give a direct boost to the economy by transferring money from people who won't spend it to people who do.

Taxation in the States is at an all time low, and a handful of decades ago the US has much higher taxes at the top and much bigger growth. So there's also plenty of historical evidence supporting the case for redistribution.

There are also a number of moral arguments (which I think are much stronger).