(Extremely stupid question) Why do people spam?

I’m not talking about spam emails, I’m talking about things like those spam threads and posts we see almost everyday here!

Why do people take time to spam? I mean, no one is that dumb to fall prey to those: “X is a natural weight loss sustain which will help you lose weight in a week without excercising”. Moreover, if you spam by emails, you have more chance to get your advertisement sink into people’s mind than randomly post spam things like this. Why take time to do something so time-consuming and useless like that?

This thread concerns about both Fluther community and the internet in general.