Why are cops able to turn off their body cam?

Whenever I hear about a brutal case where the officer turned off their body cam, it just doesn’t make sense that they have that option. They’re cops, in what instance are they ever turning off their body cam footage? It seems like a huge design flaw that doesn’t really serve a purpose. Even if it’s for a private moment, can’t that footage just be deleted? Being able to turn off your body cam footage seems to me like a power move. Genuinely wondering what the purpose of the button is here