Why do we have Wall Street?

In a word, economic development.  Economic development means starting up new companies, building new factories, opening mines, building dams, launching ships and 747's, expanding production facilities.  All these things need money, often a lot of money, and it takes a long time from startup spending to making money from sales.  Might be years between first spending on production facilities, advertisements, research and development, and first accounts receivable from sales.
   You gotta raise money to get a startup company up and running.  The nicest money raiser for start up companies is to issue stock.  They can pay their initial employees largely in stock.  They can pay their initial investors in stock.  Stock makes the stockholders into part owners of the company.  They get to vote on the board of directors, and they are entitled to a share of company earnings, (dividends).  A company can use stock, which it can issue for merely the cost of printing stock certificates, to cover a lot of expenses.
   The stock market (the heart of Wall Street) exists to give value to stock.  Stockholder's know that they can convert their stock into cash, right now, on the stock market.  That gives the stock real value, far more value than the promise of future dividends will give it.  Lotta startups never pay dividends, they tell the stockholders to just sell the stock for cash.  Which works when you have a stock market in New York and other places. buying and selling every company's stock, every day.
   In short, Wall Street provides the money that makes economic development happen.  The stock market guys are pretty good about picking winners and losers, a helova lot better than any gov'mint snivel servants will ever be.  The stock market steers investment money into winners, and shuts off money to losers.
   In short, we need Wall Street to keep the US economy growing, and hiring workers, and producing the accustomed flood of low cost, high performance products into the consumer market.