This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
From: Peter "My views are not to be confused with those
of a rabbit librarian" Alway (firstname.lastname@example.org)
Sounding rockets that are aerodynamically stable are often spun at a
slower rate that insures that any off-axis thrust will cause the
rocket to corkscrew, rather than follow an arc. The corkscrew may
be subtle--but it beats an equally subtle arc. A sounding rocket
that naturally describes an arc with a 20-mile radius due to its
asymmetries cannot reach higher than 20 miles. But if the rocket
is spun through 360 degrees every few hundred feet, the
20-mile-radius arc turns into a very suble corkscrew.
Imagine the modeler puts a very slight misallignment between the
forward and rear fins of a sidewinder. Suppose it's just one degree.
also suppose the fins are 1 foot apart. the rocket will naturally
arc in a circle with a 360-foot circumference and a 57-foot radius.
That's instant doom! make the error half as bad and you are in
trouble. But if the rocket spins every 10 feet, the path will be
a generally upward corkscrew, less than ideal performance, but
a safe flight.
So with model rockets, a spin on ascent is a good way to make a
marginal or asymmetrical model safe. Estes used to sell a space
shuttle orbiter kit that had a spin tab for this reason, and the
old Astron Space Plane had spin tabs as well.