My theory: its state can be detected by touch alone and toggled without having to look at the device. This is very convenient when it's in one's pocket or bag,

My theory: its state can be detected by touch alone and toggled without having to look at the device. This is very convenient when it's in one's pocket or bag, especially when it's already making noise. There additional benefits: you get one additional hardware button (or in this case, switch) that can be remapped to other functions. iOS does this for the screen rotation lock, where it again pays off immensely: when the screen is already in an awkward rotation, not many people will want to poke around an on-screen menu to get the screen to re-orient.