Why do large companies go public?
To get large financing. They emit an initial offer of shares (IPO). That initial share price is calculated by an investment bank on the value and potential of the company and on the market response to preliminary approaches (or road shows) for less well known companies. That is the ONLY time the value of a stock is based on any type of reality. After that, its a free for all and the price is only decided by how much people are willing to bet on its future value. As far as the originating company, the only thing it gets out of it is the initial share sale take (minus costs) and whatever the value of the stock it kept is worth. On the bad side, they get a boardroom filled with investors or their reps. And unless they still have controlling interest in their company or some sort of contractual obligation or special share types (non-voting), they dont own it anymore and are just an employee, at the mercy of the new directors.