Advocate of the devil here: Would it then not become just a numbers game? This post is liked by x amount of people on this forum? What does that add exactly?
-Shades
The point of it would be so that you could express thanks or approval or give some form of feedback without having to create a new reply which would therefore ping everyone who has signed up to be notified of new replies. It also helps keep threads on topic instead of having a bunch of superfluous "thanks" or "nice" replies.
This would be especially handy, IMO, in threads such as the latest Steam or GOG giveaways.
As things are now, I have to choose between no reply at all (which can give the OP the idea that nobody cared about what they had to say/share) or possibly bothering a bunch of people with notifications just to say something like "thanks" or "I enjoyed that" etc. Often times I choose the former.