I think
1) in part, it's an outdated reference to the way the site used to look, but still a good reminder to Like helpful posts
2) when a thread is started as a "Question" (type=Question), then there are Up and Down buttons where you can upvote or downvote an answer. Unfortunately, the concept of starting a thread as a Question is rarely used, and thus the system is largely out of scope in day to day usage.
edit - as isladogs reminded me this only available on modules & vba forum
I for one am very liberal with my Likes, but I have noticed this is not necessarily the norm and in fact is rare. Some people probably feel clicking Like costs their ego a little bit or something - but I try to be very liberal with them, using them whenever someone posts anything particularly good or on-target. Who knows, maybe "karma" is real, because my like percentage is also high