Merits isn't really "important",if you actually do not mind remaining a newbie for all your days in here and definitely not ranking up in the process,then it's fine not to worry yourself about merits
But if actually you wish to at one point or the other participate in a bounty and probably try to earn, you'll definitely need merits to rank up,as only users from Jr member and above are allowed to actually put on a signature and advertise for certain projects

You earn merits by sharing important knowledge and information,in a quality and novel manner,not like it was said in the past,or copy pasting information as that will lead to your ousting from the forum

You cannot buy merits but you can purchase a copper membership rank to actually give you certain privileges,you can purchase it Here