What are stimulus generalisation and stimulus discrimination? Why are they important?

In both classical and operant conditioning generalisation occurs when subjects respond similarly to stimuli that are similar to the CS. For example, in Watson and Rayner's study with Little Albert, Albert was conditioned to fear a white rat but later came to be afraid of many white, furry objects.
Discrimination is the opposite of generalisation; that is, the subject will respond to a specific stimulus and not to another stimulus- similar stimuli don't elicit the same response. For example, you stop at a red traffic light, but go at a green one.