A ballistic helmet protects against fragmentation and handgun-velocity threats, and gives you somewhere to mount a light, a headset or a night vision device. That mounting platform is, for a lot of owners, as much of the reason to own one as the protection.
What it does not do is stop rifle rounds. No wearable ballistic helmet does — the weight required to defeat a rifle round across a curved surface that size does not exist in a form anyone could wear. Anyone claiming otherwise is selling something.