What encourages people to buy shares in the ownership of a private limited company? (please if you can, give an explanation)

I think the main reason people buy shares of companies is to make money.
Explain:
their idea is to buy low value of things and sell high. for example, if i buy 100 shares of a company stock valued at 25$ each i will have made a total of 2,500$. if in the next few months the shares increase to $50, I can sell them for more, Like $5,000, This is doubling your investment.
Hope that helped

Ok i a