Links to websites for low ranked websites could be the action of someone trying to spam your site using negative SEO to drag your site down.
Website rankings rely heavily on the quality of websites linking to them. If they suddenly get a blast of low ranked websites, it could be the competition dragging you down.
Other places will link you to you hoping that you'll see the link in your Analytics or Link Reports, and then click to their site. The moment you click to their site, they can then use malware to infect the browser (dark arts) or use the visit simply as a way to pump their numbers so they can sell more advertising (ex: I got 20,000 visits last month, you need to buy an ad).
Google is pretty smart but still relies on people to help. The disavow tool is great (if you know what you're doing), but as Artashes said, MOST sites can just ignore those spammy links and concentrate on quality traffic.
We performed a site audit a few months ago for a client with just over 1,000,000 links pointing to their site. A good majority of them were junk links, but not worth spending the time to disavow. We uncovered a Negative SEO campaign and were able to squash that using the disavow tool and 301 redirects.
For *MOST* people, you can ignore them.