I just had a look. 789 comments have been posted to this blog ... and about 20,000 spam comments. About 99.99% of those spam comments were filetered by the blog software straight to a spam folder, so no-one ever saw them. Of the 0.001% that actually made it to visibility, no-one ever read them, let alone clicked on the link, and no way bought anything.
So why do they do it?
I think most spam is a scam run by the spammer against a genuine business. For $100 (or whatever it is) they say "Your message will reach a million readers" or however many it is. But what they don't tell the business is "reach and be ignored by".
One of the common factors I hear when I contact a business spamming me, is that they feel swindled by the people who sold them the list and sent out the spam.