Spiders eat mosquitos and mosquitos carry viruses like west Nile virus that are dangerous and even deadly to humans. They also eat other parasites, insects & vermin that spread disease to other wildlife and humans. Just their main purpose as far as I know...
For 3 years, we never had a single edible strawberry in our garden. Then, last year, we suddenly had hundreds! As I was gathering all of the strawberries, I thought I saw a rock move!? Upon closer inspection, the "rock" turned out to be a BLACK WIDOW SPIDER! I was barefoot so I couldn't step on her and couldn't take the chance on her getting away and risking a painful & dangerous bite, so I scooped her into a jar and we kept her until she passed from natural causes. She was eating all of the bugs that were eating our strawberries and once we removed her now we again don't have any edible strawberries. I learned a lot from her and now we try to educate people on how useful spiders are so they don't smash them.