To me, giving a like to someone’s post let’s them know that what they wrote has at the least been acknowledged. Imagine a group of people talking and one person says something no one replies, makes any remark, or even smiles. For that person a feeling of “why even bother to say anything” can slowly creep in. For them it would be the equivalent to “talking to a wall” - saying something and getting silence in return.
I’ve had a user who, for a while, did nothing but post links to articles in various topics without making at least a comment about how he/she felt about it. It wasn’t until I mentioned that posting the links is fine, but engagement was sorely absent - no discussions - unless the user started off with a remark, posted the link, and replied to others when they responded to that user’s comment. I also mentioned liking others posts using what I posted in my first paragraph above. Finally, a back-and-forth discussion ensued… and the likes flowed! 
We still have a couple of users who read a post, reply and forget to like the post. But that happening less and less often. 