First let’s 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 don’t 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 company near by and your property is accessed by power lines already. The problem is that the power plant is no longer able to produce power or get power to you for one of several reasons. Often it is weather that has damaged the power lines that supply your zone with power. It could be high winds, ice storms, and other situations that knock down the lines. The plant also may not be able to produce any electricity or enough to supply your needs. This happened just a few years ago in the Midwest when a power company in Canada failed. The caused a cascade of “failures” in which the power companies down the line in the grid could not produce enough to completely back up the original failed plant and people went without power because of it.
So when should you have an electric generator? Well it all depends on how you want to survive when power goes out and how much you would lose by not being able to operate. Hospitals for instance must have generators in order to provide continuous life saving support. Contractors as mentioned before typically have to have generators just to work at certain jobs. If you live further away you would more likely be benefited. If you are just the type that loves to be prepared for anything than you would be happy to fire up the generator when the power went down.