What is a mandolin used for?

Taking the tips of your finger off. Every mandolin seems to require a blood sacrifice before it gets the proper respect it deserves.
That said...Potato chips. Angel hair cabbage slaw, carrot shards..as mentioned anything you want consistently sliced like raddish disks for salads etc.

Slicing apples for a superior apple pie.

If you are planning to get a mandoline, I recommend getting a pair of Kevlar gloves to use with it to keep your hands safe. That blade scares the bejesus out of me.

Pretty awesome tool for slicing. Watch your fingers and knuckles.

uniformity in slicing. good for cooking at same rate or for the aesthetics of nice slices. watch your knuckles.

A mandolin is used for playing music, often bluegrass. A mandoline is used for slicing, often vegetables and fingertips.

To consistently slice things very thin, very quickly. Some have attachments that enable you to julienne or grate.