Saves your existing aluminum or alloy rims from salt, saves the rims and both sets of tires from potential abuse from a bad tire mounting job, quick do-it-yourself changeover in the fall and spring while saving money, etc.
I suppose if you were going to take the vehicle to the tire place to do the changeover anyway, and especially if you have OEM steel rims, you could forget the dedicated winter steel rims to save money up front.