We have cats inside the house because they are loving companions, bed warmers and they eat literally anything that moves.
We never have any trouble with flies in summer because the cats fight over who gets them. Same with spiders.
I would like to have a dog but the ticks where we live are just way too bad. Many outdoor animals die each year from tick bites. Even with tick repellent they can only take so much before they get too weak.
Our cats are never allowed outside without supervision due to coyotes, fox, wolves, bear and eagles.
We also use diatomaceous earth for fleas and parasites as well as for mites on our chickens.
RE: Dear Diary: What Is The Function Of Your Pet Animals For You?