When I was in grad school, I decided to spend a summer and work on Wall Street. Though I quickly realized that it's not the place for me, it was still exciting for a summer. On my last day, I asked my MD (managing directors are basically two/three levels up from a VP) how he can justify working for a company that only cares about making money and the rich richer. He had a very good answer:

The modern world requires a lot of money. Cities need money to build roads and bridges. Corporations need money to expand their infrastructure and business and create jobs. And the rich and even the middle class have money they like to invest at a higher rate than the prime interest rate. So you need firms to create securities over these financial needs and sell them at various risk/reward levels to investors. Without the Goldman Sachs of the world, the Golden Gate Bridge would have never been built (as it was built using a municipal bond).

Granted, this doesn't justify some of the bad things that occur on the Street (like the close relationship between the IBanking and Equities), but I like this argument as most people don't see the value of these firms.