You get what you give! By meriting others, you help them to rank up and give them a stimulus to keep writing high quality posts. So, as a result we get a forum that has a higher percentage of high quality posters that makes this place absolutely better.
When you'll ask a question, if 9 from 10 was posting spammy answers, now this number will be reduced. Just see the result of merit implementation for these years. If people wouldn't merit each other, then we wouldn't see this result. I don't say that this forum has turned into goddess intellectual place but there is significant improvement in this aspect.
On another hand, I guess no one has noticed this but there are some old/new threads with high quality seo profile because of backlinks and etc. When you merit posts in such thread's main page, you get backlink from it on your profile and this makes your profile's seo score higher and backlink from your profile gets better but hope no one will merit unnecessary posts because of this reason.
Also when you more smerit on post, you are shown on top and it increases celebrity on this forum at some point