Although plenty of people do it, most aren't treating their whole yard or entire garden in pesticides on a regular basis. Most people who are using pesticides are just spot treating here and there, maybe spraying their home's foundation to keep out ants and termites and things of that nature.

People who use pesticides in their lawns will have different reasons and different approaches, but some common reasons (real and imagined, I'm not defending the practice) are typically to control pests like fire ants, Japanese beetles, yellow jacket wasps, termites, fleas and other parasites, and many other things that are region specific.

And honestly, some people just don't like bugs. I think that's ridiculous, but it's way more common than you might think. Any tiny creature in their house warrants the nuclear option. A wasp nest on the underside of a deck terrifies them.