To bestow street cred upon a nerdy white boy/girl. This street cred cannot be contested, and remains valid until it is removed by the person it was given by.
Buying products, usually nail polish or other makeup, and shipping them to people in other parts of the world where these products are not available or are prohibitively expensive. Believe this comes from the idea of a pack mule that carries goods from one place to another.
I wouldn't have any of these MI-NY holos if it weren't for Maria muling them for me from Italy.