Uniting colonies is a time-honored method to achieve various results in beekeeping. This is an important skill to learn.
Contributor Dave Cushman provides the following information about this technique:
There are many different circumstances when it is advisable to unite groups of bees, such as:
Reducing colony numbers, where manipulations have resulted in extra colonies, or the beekeeper is downsizing.
Making a colony from two or more that is strong enough to have a chance of wintering that wouldn’t survive on their own.
Making up for a shortage of queens at the end of the season, where there may have been failed matings.
Uniting (stacking) a colony has been queenless, had a drone layer or laying workers onto another healthy colony.
Re-queening a colony.