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These are used to fine tune the axial position of the center of mass Cm for the rocket.

They can also used to both improve flight stability in strong crosswinds and  also to improve the nose cone separation point.

The other parameter that can be changed to obtain  the required separation between the centers of mass and aerodynamic pressure. Is to reduce the  height of the center of pressure measured from the base of the rocket. Cp

This can be achieved by changing the lower profile of the rocket to increase its surface area. 

One example would be to increase the fin area without increasing the aerodynamic drag force significantly.

This needs to be done so that the total surface area exposed to the air or fluid flow over the rocket surface is not too large.

Although the rocket needs to be as light as possible the  ideal would be for the Cm  for the empty  rocket mass to be near the front nose cone.
 A little experiment that will help illustrate this is to manufacture some simple sling launched arrows.

Modifying the fin area and center of mass to produce the best flight length.

Note all the modifications made and record the flight distance obtained.

Discuss what combination provides the best result and explain why.