Ballast is often used to adjust a rocket's CG and target altitude. It's especially useful because as parts are added and removed from a rocket, the mass and CG can change drastically. In the case of IREC, where there is a target altitude, ballast is essential to be able to get closer to the target altitude for maximum points. The CG position is critical for rocket stability. Ballast is unnecessary functional mass; it does not contribute to other subsystems.
Ballast is usually some kind of dense material that is easy to discretize. This way, it is easy to adjust for specific flights. Lead shot, steel washers/disks, sand, and clay are good examples. The ballast must be strongly secured within the rocket - it can easily become a falling piece of metal if you aren't careful. (PICTURE OF BALLAST DISKS) (PICTURE OF LEAD SHOT EPOXIED IN A NOSE CONE)
Typically ballast is positioned in the nose cone (to shift CG forward) or near the motor section (to shift the CG backward). These are locations that usually have a bulkhead that ballast can be attached to. For smaller rockets, lead shot can be poured into the tip of a hollow nose cone and epoxied in place. However, this is less optimal because adjusting the ballast requires either a new nose cone, or lots of unnecessary effort.