Some places (especially international ones) strictly enforce the "bathroom is for customers only" rule so I have to purchase something in order to be given the key or the code for the loo. I ask them to keep it for me behind the counter after I've paid as I don't want to use the loo with a hot cup of coffee or an ice cream sundae. The international locations don't always have something small and inexpensive I can purchase such as a tin of mints or a cookie that I can stash cleanly in my bag.