What is the function of the Property URL?

What is the purpose/function of the URL value when creating a new property? I noticed that this has no affect on whether or not a site actually loads the library and fires/uses the tags and events associated with the property. I.e., you can create a property with URL www.fake.com and then load the library file into your www.real.com site and the events will fire on real.com regardless of what you put in the URL value of the property. 

If this doesn't control what originating sites are allowed to use the rules, then what does the URL do and when is it referenced or used?