It locks the pixels, so when you paint over it'll only paint onto the areas you have added color to previously. I find it easy to make neat looking, consistent shines on objects that are apart from eachother by locking layer and then pulling the brushstroke over all areas. I hope that made sense, it's a bit difficult to explain. *__* But basically by locking pixels you won't be painting into areas that are transparent.
I kinda gave up on digital drawing a long time ago, since I never really liked ANYTHING I drew on the computer... but now looking at these methods, I kinda want to have another go