Protection is one of the reasons why people join gangs

Membership in Los Angeles street gangs increases in response to violence rather than precipitating it, suggesting that people join gangs for safety when the state does not provide it (Sobel and Osoba 2009). Consistent with this finding, protection is a common reason given for joining a gang (Melde, Taylor, and Esbensen 2009), and protection was found to be one of the most common ser- vices provided to members (Jankowski 1991, 122–23).

Source
Read the Sobel and Osoba paper
Read the abstract of Melde, Taylor, and Esbensen
See part of Jankowski's book in google books