They are supposed to prevent fretting of the crank by the pedal shoulders. They seem like a good idea and I always wanted to get some, but the only ones I could find are the TAs on the Harris Cyclery site and they want $4.95 for a pair.
I found some on eBay for $2.75 a pair and $.62 shipping. I bought enough for all of my bikes and shipping was still $.62. They're stainless steel, so they won't rust like the TAs do. I installed a set today and they're a perfect fit. They really only show at the flats in the spindle. They'll protect the resale value of a new crank and hide the ugly marks on a used one. They fit French pedals, too.
The seller is ventoux. If you want some you better hurry. He only has two sets left.