Purchasing common stock is often described as making an investment in the company. In exchange for this investment, you receive shares of the company's profits and, hopefully, you see some capital appreciation over time.
This explanation would make perfect sense if you were paying the company in exchange for a share of the ownership. But since you're simply purchasing a share from some random person, how does this purchase help the company? How is a purchaser "investing" in the company, if the company will never see the money the investor pays?