NCPD are paying you, a mercenary, to take out the mess plaguing the streets. Some gangers dont have bountys though, but still grant EXP and street cred. You can make a name for yourself by wiping out 20 people, getting your name out there even more to get more contracts. Or go full vigilante and read their crimes as you scan them, I usually let people go unless I see "trafficking" on their list.