Because fat serves other purposes within the body besides being stored energy. It's kind of like asking why one needs to drink water if they're not going to sweat and exercise. Body uses both for other different processes. Fat, specifically, is needed for joints, organ protection, regulate body temp, etc....It's why some people struggle so much to go from a decent stomach to a ripped six pack. Their body is resisting the severe fat loss because it needs it, eventually it it has to give in though, and you've forced it to do something that goes against it's survival instincts. Its different for everyone though, so its hard to say you can't go past a specific percentage.
Most people will have a fully defined 6-pack between 8 and 10% BF, which isn't unhealthy by any means.
Like i said, it's different for different people. You can't say everyone below X percentage will be at an unhealthy level, just like you can't say everyone at x percentage will already be ripped. That's why i never mentioned any numbers and just said you shouldnt be too low and have some fat left. For some that may be at a different % than others... If i had to throw numbers out there, i'd say in general for most men 5%, maybe 4% and under would be too low. For women is higher, since they naturally need and carry more fat...for them it's probably around 8-9% and under
