Everyone better be or they will lose the readers.
It's important for an author to know these things about a character (and not just main) for that character to be real on the page. It's not usually necessary for the reader to know. But all the tidbits can work their way into the book to enhance the reader's understanding. For instance, the character is limping on a sprained ankle:
"You want a hydrocodone for that pain?"
"Nope. Ibuprofen works fine for me."
Just that exchange can explain the character to a reader better than three paragraphs of straight description.
Jeff
Totally agree with your point, but that exchange might not be the best example. Hydrocodone just makes some of us itch.