Why join a wiki, when the permissions are independient?

Hi, thanks for such great software. I am trying to understand, what is the reason for join a wiki?, if the users can see it anyway.
What are the advantages of a user for joining a wiki? I find hard to understand, if the user joins, but it has no permission, can not se anything anyway, if has permissions, then it can see the wiki whatever is joined or not, it is only to get update notifications? or there is something i am missing out.
I found very good documentation about join to a wiki, accept invitations and other thins, but i could not find some place where they made clear what the user get when is joined to a wiki.
Thanks in advance for the help!

See discussion at Call for usecases around membership feature of subwikis