In general, I can think of two good reasons.
First, if it's cold outside and you want to heat up your cab, your engine won't get warm enough just idling normally.
Second and probably most importantly, it would allow you to use high-power electrical equipment. Winches, worklights, etc draw a lot of juice, and increasing the idle allows the alternator to supply more power. Some folks also have PTO's installed on their trannies, and increasing idle speed allows these devices to work better.