Question

Why does a rough surface have greater friction than a smooth surface?

Answer 1

Because on rough surfaces irregularities of surface cause an interlocking due to which it creates more friction and as smooth surface has very less irregularities so it can move easily and hence less friction.

Answer 2

Whenever a body moves or tends to move over the surface of another body , then a force comes into play which acts parallel to the surface of contact and oppose the relative motion . the opposing force is what ???
off - course - FRICTION force !!!!

According to F.P Bowdon :-
due to contact , binding force start to act on in contact surface this binding force increases when roughness of surface increases, hence, for increment of binding force roughness must be increased , but we know according to definition of frictional force binding force ( opposing force ) is nothing , this is frictional force .
Hence, Frictional force is greater in rough surface compare to smooth surface .