Because you need to push your skills and equipment 200% past were you think you could be at. What if SHTF and you are stuck fighting with your carry gun to get back to your family? Can your gun get dirty and still function after 100 rounds? I like to know the limits of both myself and my equipment.
Oh, not saying that I don't bring them to the range to see what my weapons can do, but since I can't afford more then a couple hundred rounds at a time, hard to gauge where they will fail then. And I am way too OCD to ever leave dirty weapons in my house. Heck, weapons cleaning has been drilled into me that its the first thing you do after the range, so your weapon is ready to go ASAP