First lets define an electric generator. In simple and obvious terms it is something that generates electricity. In effect it is its own power plant. Most often it is run by a gasoline engine but it only requires power to spin a magnet inside a coil of wire. So you could harness the power of the wind with a turbine or use the hydroelectric power of moving water.
So basically you need an electric generator to produce electricity apart from what your local power plant is producing (by the way they are just super sized electric generator(s)). What kinds of times would these be? Well say that you dont have access to the local supply. You are either too far away from a power company and their wires or you are on land that has not been accessed by power from the power plant that is nearby. Lots of times carpenters and other tradesmen in the house building industry will have generators on hand incase the power has not been turned on yet. Other times it is hunters or other outdoor sports enthusiasts that have equipment that can be run with electricity including lights, coffee makers, griddles, TVs, etc.
The last situation that came to mind is when you both have a power...