Fleas are everywhere in nature and multiply incredibly fast. They get on your pets when they're outside and join you in your house when you come back inside. Fleas eat blood, so they'll bite you and your pets. Ticks can be dangerous to humans, as they can carry diseases like Rocky Mountain Spotted Fever. They'll grab hold of your pet while its outside and come inside your home.
Spiders eat other pests.
That's why they like coming inside your home, if you have other pests. Spiders won't bite a human unless they're provoked, but if one does bite you it can be painful. Silverfish like to live on your clothes, especially clothes that are made of non-synthetic fibers. They'll also get into books, papers, and wallpaper. They like to eat at night, and can destroy important belongings.