Yes. People leave comments just for the back-link ot their site. it's a very common and very popular SEO method that's bene used for a long time now.
Basically, if a comment comes from someone that you don't know, and it looks like it doesn't have anything to do with the post, then there's a 99.9999% chance that it's SPAM. Even when it does look like it's a good comment it can still be SPAM. It can be tricky to tell the difference.
The bigest problem is that there's a lot of automated programs out there that can register, log in to your site, post comments and do more, and they are used to SEO "professionals" all around the world to boos ttheir own back-link count.
For myself, I treat anything that has a link in it as suspicious unless I know the person that posted that comment for real. If it looks like SPAM, it's SPAM. Anything that is like the ones that you are getting (incoherent, don't read properly, links back to a common site) should be deleted and have that users account deleted/baned.
Just as a note, this isn't just a WordPress issue, This happens on almost every platform out there.