Merit is basically a special key to up everyone profile rank. Which you will get only based on your post quality and informative .
Now let's talk about its benefits. This will up your rank.  Although activity is also required to rank up but your activity will automatically increase after a certain period of time after you post something.  So it's not a big problem. If you get many merits in a short period of time, it makes everyone think that you are a wise person . If your rank is Sr or above.  And if you have a consistent of regular getting merit history, you've more likely to be accepted into Good and Longterm Signatures campaign And these have many more benefits like these. hope you understand why need merit and what is the benefit