Here in Ireland, we always clip our horses in October, as the hunter trialing and hunting season starts then. If you think about it, its like us when we go ice skating: We start off with a tonne of layers, because we are cold, but after ten minutes of skating, we feel roasted, and need to peel off a few layers. (Everyone thinks I'm nuts for going ice skating in shorts and a tee shirt, but when they try it they understand!)
So with horses, when clipped, they usually start off with a buck or a frisk or two as they are cold, then they start to sweat as they warm up! This is where the clip comes in... Its essentially like peeling off your jumper and just going in the t shirt! Also, apart from keeping them cool, it helps the drying process if they do sweat. A horse with a thick winter coat takes far longer to dry than a clipped horse,consequently leading to a chill.
It also looks smart :)
If your horse does little, or no work he may only need a bib clip. If he does slightly harder work (like your horse) a blanket clip could work. For horses working hard, hunting and hunter trialing all winter, hunter clips would be good. And for extremely hard work, a full clip might be needed!