Reducers allow you to shoot 20 gauge, 28 gauge or .410 gauge shells in a 12 gauge over/under shot gun. They are primarily used in skeet shooting, not trap. A good skeet shooter competes in all 4 gauge competitions. The smaller the shell, the less pellets, the harder it is to break the bird. If you are serious about skeet shooting, plan on spending $1000 or more to buy tubes, such as brilleys. A set allows you to have a full length tube in your barrel for each caliber.
Jeez, at a grand a pop, I'd rather just buy another gun in the different caliber.