You mean aside from exploration with the idea of discovering something new that can be turned into a profit?

Shipping.

That's exactly what the Nostromo was: a shipping container. They were hauling goods from one planet to another for some (profitable) reason.

What we know now, however, is that a great deal of that could likely be automated / not require a crew. But even then: the Nostromo was massive with what could amount to a skeleton crew (half a dozen?). Container ships even today have more than that on them.