Why did they actually introduce measures and what are its main functions?
Because spammers could create many many accounts and spam with them to gain higher ranks and activity. With merit its impossible because you need quality posts not spam posts.
One problem with that is, people should reward quality posts which is not always the case because they don't know what merit is here or are just lazy.
I haven't been on this forum in ages. I assume merit is similar to the Facebook 'like' button where some members can give other members merit for good posts?