In his book on electrodynamics, Griffith talks about the magnetic field outside a solenoid. Firstly instead of dealing with a typical solenoid with closely wound loops, he instead works with a cylinder with a surface current that has no z-component. To get the angular component of the B-field outside the solenoid he takes an Amperian loop that is concentric with the solenoid. Then he says that no current flows through the loop and therefore due to symmetry the angular component is 0. I would like to know why he chose to work with a cylinder instead of a typical solenoid. I attached screenshots from the book below.