What's the point in getting users emails?

Can you use those emails to send them offers and make money that way?

If so, would they have to consent to it?
For example, checking a box that says "send me offers".

(Sorry if this is an obvious question. I've already searched Google and didn't find anything specific.)

Edit: Well.. I think I know the first answer. But the second answer still applies.
Again, sorry. I just code, so I'm not aware of these marketing techniques.