Because they believe that the company is likely to be able to eventually generate revenue, and then become profitable, and then become very profitable, and finally be acquired by a larger company which will be willing to pay handsomely for this now very profitable company.

Since the investor is purchasing a portion of the company during its earliest, pre-revenue days, the price the investor is paying reflects that, and is very low….let's say $X.

But when the company is sold several years later (say, in 5–10 years) the price the acquirer will be willing to pay will be much higher, say 20 or 30 times $X. That means the early investor will have made a 20 or 30 times profit on the initial investment, in exchange for taking a risk and funding the company when it was still in its pre-revenue stage.