Many people do not respond to spam. Ever. They will recognise it straight away, and report it. They never click a link in any spam email. So when you send these people spam, you’re effectively sending it straight to the index. These are the people that you as a spammer do not want to be dealing with. You want to deal with people who will indiscriminately open email, click links; who are gullible but trustworthy.
So it benefits you to find out who these people are, and send spam only to them. If you have done your job properly, and you can match up each site access with an email address that you sent your spam to, then you can figure out which email addresses aren’t resulting in clicks, and dispose of these. That way, the filters will evolve less quickly against the patterns of your emails, and you have to change your strategy less often to escape the filters. You get an easier life. Good luck!