Bee Swarms are the natural way that bee colonies multiply to increase their numbers. In the world of bees, an individual is not the important entity, as most bees cannot reproduce. The important unit is the colony, which contains a queen (or occasionally multiple queens) responsible for laying eggs which the colony as a whole hatches and raises.  It is therefore important for the survival of bees, that colonies "reproduce", which is done by the bees splitting into two groups, one of which stay in the hive, and the other departs to find a new hive.