I use a strip strike for two reasons:
1. If I miss the fish, the fly is still relatively close to it and the fish or another one close by might still take another whack at it. Setting the hook by rearing upwards on the rod (like a spinning rod) will pull the fly away from a fish that misses, and sometimes (especially topwater) completely off of the water.
2. The strip strike is a straight line connection that usually gives a better hookset. I usually strip with my left hand, and for bass fishing I also rear backwards (not upwards) with the rod.