Sometimes you really need to make sure there is no power flowing through your device, which you can't be sure of using software when you are trying to fix software problems. Of course I should have said recording devices and networking, rather than just cameras.

Also, true power off guarantees nobody is remotely activating eavesdropping on your device.

Lenovo have a battery disconnect bios option specifically for doing service work.

Maybe the concern there is devices appearing to be turned off but really still running in low power mode? The only way to be sure is to physically interrupt the power supply.

The people writing the software have a vested interest in your battery wearing out so you have to buy a new laptop