To better hold onto a memory of some one with atributes you admire.
I'd say it's more of a way to prove it or just a souvenir. They have something original and personal from someone who is typically admired. Personally, I don't really see the point.
These days some people do it for the money they think they can get selling them, but traditionally I guess it's a way to feel you have some kind of connection, however small, to someone you admire or respect