It's for protecting people. There are members that go on patrol, and they get weapons if they want them so they're kinda picky about who they let do that. And then some people have healing skills, so they can make people better if someone does get hurt. And then, if you wanna join but you don't wanna do those, you can be one of the dispatchers which is a really important job, or just do office work and run errands.
[babbles on~] The headquarters in the first sector, you can find your way on your map. Kururugi Suzaku is in charge of everything, and he can help you even better. But there's a special test, so bad people can't join at all.