If an investor buys stock in a company he is effectively lending the company some extra capital which that company can use to employ people and to generate wealth for all concerned. That helps the economy grow. The investor is rewarded by the increase in stock value.
That reward also compensates for the risk that if the company screws up the stock price will fall and the investor could lose a lot of his money.
The tax concessions are a way of encouraging wealth to flow as investment into the parts of the economy that benefit all of us. At least it seems to me that's how it's supposed to work, although I agree the system is far from perfect.
