You don't have to retire to thank people.Well people will wonder what happened and some people feel they owe it to their their fans to thank them for all their support.
And most musicians, painters, authors and other "free lance" people I know of just fade away but most often pop up again with some shows (or corresponding) in the future though.
But yeah, I can probably see one reason to announce an retirement. That's for people that want to see your last show to make sure they don't skip it in hopes of you being able to go at some future date.
(g)ay - unit