In this article, we'll take graffiti art to mean wall art drawn with spray cans in public places. This is necessary to avoid confusion, for graffiti art can also be taken to mean an act of vandalism in a very general sense.
But when I was taking a closer look at the origins of graffiti art, it appears that this art form didn't appear out of nowhere. In fact, it has risen out of a need for expression from people who didn't have the money and the means to participate actively in both the art and the music scenes.
This urge or need to express became so strong that these people started creating their own art and music using what was available to them. And not much was available to them. So they turned to using their mouths (beat-boxing), scratching on tables (to simulate the scratching of records) and invented rap. Hip hop evolved out of rap music.
At the root of it is rebellion. There's something that these artists are unhappy about, and they're letting the world know about it. For instance, if you're a fan of rap music, you'll often hear some angry lyrics. It's not within the scope of this article to discuss the source of anger. For now, it's good enough to understand that rap and hip hop music stems usually stems from anger.
Similarly, for graffiti art, rebellion is made known by creating wall murals in public places. Doing this is illegal but yet, many have done this and it does appear that graffiti art is now out of control.
Well, understanding the culture where hip hop music and graffiti music come from (and they come from the same source), one would guess that graffiti art could be the cheapest form of advertising (in fact, it's free!). It's the ultimate word-of-mouth marketing for the hip hop artists. And in essence, since both art forms represent the same dissent voices of the people of that culture, who's to say or judge it's wrong to create graffiti art?