The correct answer to this open question is the following.
Unfortunately, you did not attach any context or references t answer the question. So we are going to answer in general terms.
The motivation of the people who are into graffiti is to express their emotions through this form of street art. For them, graffiti is a form of communication that relates to their culture, street gangs, belief systems, race, gender, and social issues.
The impact of this graffiti on the viewing public is twofold. First, it could be understood as a form of communication or street art that wants to convey a particular expression of pop and street culture. On the other hand, there are people that graffiti is a social problem and that people should be prosecuted for damaging the walls of the city, public, and private places.
