What is the advantage of useing a 13X6-10 prop over say a 13X6?

I have seem them before but never thought to ask. Today when I was in the LHS I did ask but then I was reminded that nobody in there flies. There should be a law that requires having an R/C flyer in a store selling R/C planes.

OK, forget that law quote. I don't want to start a conversation on constitutional law. I just want to find out if I should consider using a 6-10 prop or not. I have 2 and 4 stroke engines, ranging from .15 2 stroke up to .91 4 stroke.