The Celtics decided to take a gamble on a player whose career is probably over. Miles has missed the past two seasons plagued with knee troubles. An independent medical examiner proclaimed the injuries "career ending" in severity. Miles is only 26, and a former 3rd overall draft pick. So if by some miracle he's able to come back, he can contribute to the Celtics' quest for ring #18. It's a no-lose acquisition, sort of like signing Bartolo Colon. Only there's even less likelihood that Miles will play.
Maybe Miles keeps getting his knee hurt cuz it's so much of a strain to get in and out of his car:
That actually is his car, or at least it was last year.