Plain and simple, this was started by the Mexicans. It's their way of saying "Man".. This is due to the Mexican accent converted to English. Now people say 'mang' because it's (1) different and (2) humorous.
made popular by Al Pacino in Scarface, "mang" is the term for "man", but skewed into sounding like "mang" when said with the cuban accent of "Tony Montana"
A fun way to say "man" usually when talking lightheartedly with your buds (mangs). in certain social situations "mang" can be quite addictive to use but it seems impossible to overuse it. Also made famous in the muddy waters song "mannish boy".
Andy: hey mang!
Stefan: oh hi mang!
Andy: ju got any butter mang?
Stefan: nah mang.
Muddy Waters: I'm a MAAAANG! I'm a full grown Mang. I'm a Hoochie coochie Mang! (and so forth. great song)