Those are good questions. I don't have the answers. It's the same with junk email. I often ask myself "Does anyone buy their junk? Does anyone even read their junkmail?" I suppose the answer is that people are people, and some people cannot help themselves from clicking on links that they don't know where they go. Ultimately I suppose if just one person clicks on the link, it is worth it to the spammer.
Anyhow, you did the right thing by reporting it. Thanks.
We use a number of techniques for stopping automated spammers, but they don't give up that easy. Most the spam you see here is posted by humans, earning pennies a day in third world countries. Doesn't sound like fun, but it's a lot safer then stripping a retired oil tanker for scrap metal, by hand.
They don't care if anyone ever clicks a link. It's done for the benefit of search engines, and raising their search engine result positions. Basically, they hope some of the good reputation from this site gets passed to their own. Again, there are techniques we use to make that less likely, and Google / Bing are always tweaking their algorithms. The fact remains that a link from another site to your own, is site is still the most important factor used by search engines. So, spammers will spam.