In a natural, balanced ecosystem, insects are not good or bad. The idea of beneficial only occurs when people establish goals. In farming, the goal is to raise selected crops. Insects that get in the way are classified as pests. Insects that help the growing process become beneficial.
Ladybugs are considered beneficial because they eat large quantities of aphids, mites and other insects that feed on various plants. Bees are beneficial as pollinators, facilitating propagation and fruit production for many plants.
Encouraging beneficial insects, by providing suitable living conditions, is a pest control strategy, often used in organic farming and organic gardening.
Companies specializing in biological pest control sell many types of beneficial insect, particularly for use in enclosed areas, like greenhouses.