Why do you need emergency generators?

What is a emergency generator, it is a back up generator for in case you loose power during a storm or even a earthquake. An emergency generator restores your power but for a special limit of time depending on how much gas or energy it has left, how do they work, well they work like this, if you loose power they kick in and bring your power back for a limited time because of the gas and depends on how much gas is in them.