what's the deal with...

...cooking spray?

Lots of the recipes I have call for it, but I don't have it. It's been at least half-a-year since I've had any. We use the oil painting method here at La Casa B -- I put a bit of vegetable oil in a teacup and I let Pic use the pastry brush to coat whatever we're coating. Are there times this is going to be something I regret? Is cooking spray really necessary? I don't have a can handy, as I said, so I can't check the label, but what's in it anyhow?

Thanks much for your insights!