They can do quite a bit of damage to a lawn in large numbers.

Not very :-)) Blanket or absolute statements about gardening or horticulture seldom are. You may or may not have grubs/insect larvae in your lawn or soil and they are not always harmful if you do. And some grubs/larvae DO eat plant roots, although usually not to the extent that they will kill them. Most often it is the adult form of the insect that bothers trees and shrubs and that damage is typically on the leaves or fruit rather than the roots.

There are all kinds of myths out there about grubs in lawns and they can be a bad thing if the population gets too large. Mostly they are lees of a problem than most people make them. Many people will tell you that grubs attract moles and they are a food source for the moles but not the primary one which is earthworms. As gardengal states above grubs eat plant roots, which is why you find them in lawns, and flower and vegetable gardens, but they must also eat other things which is why they are also found in manure piles and compost heaps. This article from the University of Maryland is quite informative about grubs.

https://extension.umd.edu/hgic/grubs-lawns