How do the ratings work?
View someone's profile, and if they have ratings enabled you can rate them. You can only rate someone once.
Why would anyone want to rate me?
They might like what you posted or they might hate it. Or, asking about ratings usually attracts people to rate the poster.
Why bother with all this ratings malarky?
No idea.
Does it serve a meaningful purpose?
None whatsoever. Many people don't even know if 1 is the highest, or 10. (By the way, it's 10.)