How do skirts aid print bed adhesion? In cura one of the options under "build plate adhesion" is "skirt", which seems to simply print a loop around, but not touching, my print. How is this supposed to help my prints stick to the bed?
These skirts they don't contribute at all to help your product adhere better to the build plate other than priming your nozzle so that it is ready to lay down filament for your product. 

A skirt does give a good indication of the adhesion prior to printing your product, if the skirt does not adhere, maybe it is a good time to stop the print and re-slice with different options or fix the bed level.