While the low roll off is covered quite well here already (although, I don't neccesarily agree that it is needed in every case some suggest is should be used in, nor do I think it always makes a difference in certain applications....as Bruce like to say, YMMV) I think one thing needs to be clarified about the -10dB switch.
It WILL not "protect" the mic from high SPL on the diaphram! That diaphram is going to move no matter what! Switching that switch on or off will do NOTHING to stop that!!!
The only "protection" you can provide your microphones diaphram is to either move the mic farther away from the sound source, or to lower the level of the sound source. Luckily, most microphones can handle quite high SPL's, usually well over 110dB. I can tell you this, if the source you are micing is more than 110dB, you WON'T want to be the guy having to go and move that mic!!!
But generally, most things do not get that loud. Maybe the snare drum of a very heavy handed drummer, but I find this to be rare.
It is used to lower the mics output sensitivity, attenuating the output of the mic so that the preamp may work in a more effective level of attenuation. It does nothing more and nothing less. If the microphone is outputting too much voltage, the mic preamp will have to attenuated that much more. With heavy attenuation, mic preamps start to behave quite strange. So, by lowering the output of the mic so that the mic preamp doesn't have to attenuate as much helps keep the preamp working in a range where it has better control over the amplification (preamps DO have a "sweet spot" in terms of attenuation. It is never in the extremes). I am sure that skippy can shed some more technical light on this subject. I just learned long ago (through a LOT of trial and mostly ERROR that you don't want to have the mic preamp attenuating too much if you want better control over levels. This applies to recording as well as live sound by the way....)
Just a little clarification.....Knowledge is good!
Ed