They're annoying, but most spiders don't harm anything. Spiders don't like to be exposed. Some spiders are poisonous, but most aren't. Indeed, many Spiders collect and feed off bugs. One spider may lay eggs, bringing you more spiders.
Spiders are mostly solitary.
Spiders have a generally bad reputation. Most people try to squish spiders when they discover them. Spiders are hard to find when you decide to look for them. Anyway, you get the point. The more I think about it, the better it seems to fit.