Everyone put the signature into profile,Can anyone tell me Is this compulsory for merit and rank or how important for rank?
No it's not needed unless you'll be joining a signature campaign. The rank, you can increase by increasing your activity but the merit... not as easy.
Bringing the signature in which ICO you represent is necessary if you joining a campaign every project has its own regulations you ought to follow and need to comply. Making quality posts that can be beneficial to our community is one way you can earn merit in the forum, let the other member especially the senior member will be impressed by your post and be thoughtful to yourself you are judged here through your posts.