It always conveys a message. Recognize, read, record and remove the graffiti. There are other types of graffiti. Tagging is usually very artistic which is quite different from the messages conveyed by gang graffiti. It is important to pay attention to both types to know the difference.
Gangs develop for a lot of reasons.
Most commonly they start out as a social group. The gang develops when criminal activity begins. This criminal activity is often larceny or assault used to prove worthiness to be included in a gang. Kids join gangs as a replacement for the family. Kids join to belong.