History: Hip Hop Culture
During the 1980s, while spray paint and stencil graffiti were growing as art forms, hip hop was flourishing as well. In fact, the two phenomenon were growing off of each other.
Hip hop artist Afrika Bambaataa, who many consider colloquially to be the godfather of the genre, is known for joining together the "four pillars" of hip hop culture, one being graffiti. (Source)
As a part of hip hop culture, one could argue that graffiti gained legitimacy as form of art; graffiti thus became an outlet of expression for hip hop enthusiasts.