Why Golf ball's surface is not smooth?

You'll find your answer here:
http://entertainment.howstuffworks.com/question37.htm

The golf ball generates more drag, which doesn't make sense you may think, as it won't travel as far as a smooth sphere.

However, as the golf ball is both spinning and moving through the air, the turbulance is such that a high pressure area is formed beneath the ball, and a low pressure above it. This generates lift in the same way as an aircraft wing.

This is called Magnus lift, and makes the ball travel further than a smooth ball would.

For more information see this page. (http://www.golfjoy.com/golf_physics/dynamics.asp)

I thing for aero dynamic purpose

I just detailed the aerodynamic properties for you. Do you have any other questions?