Bee swarms benefit the bee colony by increasing space within a hive and expanding the range of these pollinators. When a hive gets too crowded, its queen and up to one half of its workers leave to find a new hive. An increase in bees as pollinators can contribute to healthier gardens and supplies of bee products such as honey, bees wax and bee pollen.
Many beekeepers also benefit by capturing a local feral swarm and adding it to their colonies to increase genetic diversity on their farms.
Some beekeepers prefer to capture a wild swarm rather than purchase a box of commercially raised bees, which may have been transported over long distances. Commercially purchased boxes sometimes come with parasites, whereas a naturally swarming colony is likely to have existed for a long time and has developed a stronger immune system through prolonged exposure to local conditions.
However, when a one of a beekeeper's own hives swarms and can't be caught, the resulting loss of bees is not to their advantage.
Bee swarms are not dangerous because the bees do not defend their nest and food stores. They also feed on honey before they swarm and are less likely to sting.