It's so you don't have to carry even more tools with you when you're out in the middle of nowhere and need to change your tire

It's so you can quickly and easily remove the wheel to load the bicycle into the car (very helpful if it is pouring rain)

It's so you can pop the wheels off quickly and easily when you are packing your bicycle into a box in a busy airport somewhere in the world to fly off to another part of the world ... and then it is so you can pop the wheels back on again in another busy airport so you can cycle off into the distance.