why would i earn money for nothing?

Hi, I don't understand how investing works in a literal sense. I completely understand the process, what doesn't make sense is if I give a company money, they do the work.. grow their company, why would they give me my money back and some more because of their hard work???

And there are so many ads, "invest! I'll help you invest! make money!" I'm sure if it was actually a great tool few people would know about It much less promote it so loudly.

Any answers are appreciated.