You are right when you say that one reason is for people to post more, that's pretty much the gist of it. The site wants more traffic. But the thing that made this little corner of social media cool was it wasn't traditional social media. Adding the option to like posts just makes this shit more generic.
Most of the good posters here (and they are dwindling) don't just like every post even if it's a post by someone they like unless they actually like it and sometimes even then they won't like it. Funny, interesting or entertaining posts should get a like or not. Before you would just banter with a motherfucker and that's how you could tell someone "Liked" you posts.