They are the most numerous and diverse of all animals. Insects pollinate plants. Bees, butterflies, wasps and many others help plants make seeds and fruits. They gather pollen and move from flower to flower, which helps plants to make seeds and fruits. Without pollinator insects, there are many fruits and vegetables we would not have.
Insects break down dead things, like plants and other animals.
Roaches, flies, and many other insects that we don't like, help with this important process. Insects carry disease. This sounds like a bad thing, but it is needed to help control animal populations naturally. When animals are overpopulated, there is much disease, and the crowding helps it to spread. When the population thins out again, there is less disease.