It leaves a residue, a film, that usually cooks on to the pan like a thin rubber film.I know you are probably like most people and don't read the 'Care Instructions' for the kitchenware that you buy so I will let you in on a little secret...you don't need nonstick spray on a nonstick pan! Get it?
If you want a little butter, extra virgin olive oil, lard or type of oil for a little flavor (like sesame) in your pan then do it. Or if instructions direct that you MUST do this, then do it if you feel it is necessary. But nonstick kitchenware is already 'nonstick' you don't need to put spray on it!
Nonstick spray was originally designed for using on kitchenware that everything stuck to, in other words everything that wasn't nonstick. Try it out...my pancakes came out flawless on my nonstick griddle without any kind of oil, butter, or spray and they browned nicely on each side. THEN out came the butter and Maple syrup!
If you look for the 'Care Instructions' for a nonstick pan, chances are it will tell you "Do not use nonstick spray on this pan". It does seem like a bit of common sense doesn't it?