It's speculation on the future development of the company's performance, you aren't entitled to a share in the profit, but you often get one anyway in the form of dividends.

If you buy a stock on the open market you don't actually fund starting a real project, but you fund the continuing development and improvement of that project, it's driven by the fact that they have shareholders to answer to, and you are entitled to voting rights.

It has to have some amount of speculation, that's the driving force behind it, otherwise the whole thing would just stagnate and you have no incentive to improve a company and it will just get looted by its employees. In the big picture it just help collectively push for more efficient ways of working, and stop inefficient work.

It's not pure speculation in the sense that you are not betting on a random event like tomorrows weather in a zero sum game.