Nobody ever needs a bivvy bag. We survived thousands of years without them. I'd say they're handy though when using a tarp as a tarp set up leave you potentially more vulnerable to changeable conditions, compared to a tent. I always pack a bivvy bag for that reason. They're a handy and versatile bit of kit if things go pear shaped. I don't mean in a survival situation, I mean it can prevent an early trip home and a ruined weekend.