If this is a stupid question, I apologize in advance.

Let's say you got a 15X6.5 wheel, and you but a 7 inch wide tire on it.... or a 15X6, and you put a 6.5 inch wide tire on it. What are the advantages and disadvantages, or does it make any difference at all?

I'm asking because at tirerack.com or other tire websites or stores, they say that a 6.5 or a 6 inch wide tire would both work for a 15X6.

 

wider = better grip
wider = more weight

 

They'll exhibit different sidewall flex, shock load characteristics. Many front-driver touring cars use the same rims all around but a wider tire in front and a narrower tire in the back. This is for various reasons. The narrower rear tires will help neutralize the understeer. The wider tires in front will provide more contact patch, and better deformation angles, plus absorb torque shock through the acceleration curve.