To produce power. Check out this blog g3520 for in depth guide to CAT g3520 generators. It gives you a break down of its use and importance. Also, if you are planning on buying a generator, go to http://www.impcorporation.com . They have new and used power generators

Also check out CHP generators. For cogeneration, the engine or turbine uses combustible fuels to create mechanical movement. The electrical charges are then passed through an electric circuit to produce electricty.

This blog goes more in depth http://www.impcorporation.com/blog/what-is-cogeneration