If a bullet could maintain straight line flight, with the bullet axis coincident with the flight axis, then the effect should theoretically be zero.

However, a variety of factors can cause the two axes to NOT be coincident. In this case, imagine you're driving down the road with the window down and you put your hand out. If you angle your hand relative to the flow of air past it, you can easily feel the increased drag on your hand.

The whole point of spinning the bullet (gyroscopic stabilization) is to keep the two axes coincident through the expected/desired flight path of the bullet.