Kicker engines are extremely useful for many boating situations. They serve as a great backup if you have main engine issues; this is especially important on big water or remote locations. If you're a troller and have a large primary outboard, kickers allow you to troll much slower, run quieter, and consume a fraction of the fuel. Wired2Fish's Scott Glorvigen provides some good information on choosing the proper kicker motor shaft length given the transom heighth of your boat.