Overlap is the point. Two logs, the records they share, and how far apart the scores are.
Two listeners are compared on the records they have both rated. The match is the average gap in stars across those records, held back when there are only a few in common, so that two shared records cannot read as a perfect match.
A small gap means the records they have rated and you have not are worth your time. A large gap means you disagree, which can be just as useful.
Your profile, ranking, reviews and card are public, unless you set your profile to private, which stops anybody reading your log at all.
Your play history is never public. Messages are between the two of you.