Three questions are involved in your questions and All deserved it's own answer
(1) what's merit. Merit is the system which is being render for the growth of everyone who is in the forum, and if you are not opportuned to earn merit before your activity reached to a certain bracket of the activities of the forum rank positions such as jr member and member, full member, senior member and  Hero member and finally legendary member, you will remain stagnant until you earn a merit that worth the any of the position your activity approach to.

(2) how do you earn a merit: merit can be earn through your contribution, and it's being award because of appreciation to show that the person like your post. So liking your post is something that deals with good ideas of responding or contributing for someone conversation.

(3) the benefit. The benefit reply on my answer to towards what's is merit, because it's only means of getting to a new rank and if you don't earn merit you will remain stand see with one particular stage.