Why do spiders spin in their webs

If a spider is in it's web and gets disturbed, I sometimes see them spin on the spot frantically, why is this?

I can only assume it's a defensive move, but does anyone know for sure?
Asked Jul 10, 2014
They work very hard on webs.
If a big part of a web is missing, the whole thing can come apart.
They have to work fast, to make sure the web stays healthy and in place.

Answered Jul 20, 2014

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