These are mechanical touch sensors that are used to detect environment. They are used when you need to stop the bot after it touches some thing.
These are generally used with some whiskers like wire to detect from a far distance and mostly used in beam bots as they take very less power and give also very less power.I have seen some bots which use them for sensing the obstacles rather than using IR sensors or any other sensors as they are easier to use than ny other circuit designed sensors.
The other two types of switches are the magnetic reed switch and the membrane switch, which is much like a long button actuated switch.
