If the stack were smooth, the boundary layer near the surface would cause the airflow to slow down as it rounded the curve of the stack.
By placing dimples along the surface, it breaks up the boundary layer and induces turbulence near the surface. This prevents the airflow from slowing down or stagnating.
Since the airflow retains more energy, it is able to continue past the curvature of the stack and into the throat.
This is also the reason behind slightly roughing up the inside wall of an intake port. It allows the air to round the inside bend without stagnating and maintaining overall intake velocity.
Also one reason why airplane wings sometimes have the V-shaped vortex generators. It prevents airflow stagnation at higher angles of attack.
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