Zoo's should be illegal

I see a lot of people saying "but zoos help save endangered species" but i don't know if that's always the case, most zoo's just seem like a place to put animals so people can visit them so they can see the animals face when they look at them through bars. I just don't see a lot of help they give to endangered species, so i'm guessing a lot of them just take animals which would have survived in the wild and put them in a cage to display them.