If they have your viewing history and IP, they know what sites to MITM you on and where (your IP) to probe for further information in terms of what exploits to attempt in order to gain more access to your system, for one. If you're doubtful anyone would care about that for you specifically and therefore it makes no sense, you may be right about everything before the "therefore." But they don't need to care about you as a person to care about the potential resource of your system, which could end up being one more system in a "botnet," a hop point or exit node in their own little illegal VPN which could leave you the subject of criminal investigation for anything they choose to do while using your IP as their own. And whether they cared about you specifically at the start or not, that access may lead them to care about you specifically when they're bored and decide to sift through anything they have access to, which could potentially be anything and everything on your system. And that level of caring isn't likely to be beneficent, at best you could hope for "mutual benefit" wherein the "benefit" to you is not being used as a patsy for their computer crimes while still losing the privacy that is lost by having others watch your actions (potentially literally, if you've a camera/mic connected in any way to your router, like the camera/mic installed in just about every commercially available laptop/cell phone) without your consent or knowledge.

Don't think people would do that to some random stranger? Do you perhaps, like many, believe in the inherent good of mankind? Well, allowing that as a possibility for the sake of argument, perhaps like Brenda Ann Spencer they just Don't Like Mondays and chose to liven the day up.

The sub you really want for specific answers to the possibilities of these things would probably be /r/asknetsec/