Graffiti serves to break up the aesthetics of control. Even if it isn't explicitly anarchist, it creates a slight break with the monolithic and imposed architectures of capitalism, and has often been noted, one tag is quickly followed by another and another. Optimistically, this helps create a growing sense of disrespect for property, commodity, and law.
AG Schwarz has written about similar things, sounding much smarter than I, referring to them as "signals of disorder," here: http://theanarchistlibrary.org/library/a-g-schwarz-signals-of-disorder-sowing-anarchy-in-the-metropolis
As far as the poor public workers who have to remove graffiti, there are some anarchists who would argue this, I am sure (along the lines of the smashing a Starbucks window only hurts the employees argument), but I would counterpose to my more workerist friends that perhaps the taggers and street artists are doing their best to ensure ongoing employment for the masses. I mean, if their job is cleaning graffiti up, they need graffiti to clean, right?