Here's a funny product description from Hub Jub, across the pond:
You don't need chainring spacers. Your chainline is perfect. However, let's pretend, just for a second, that you have this... friend whose chainline isn't quite right. If your friend puts a set of these between ring and spider, they can move their ring a little, hopefully enough to shift it into the plane of the cog and correct the misalignment that's bothering you, ahem, them... Sets of five.They sell too-cool-for-skool track bits, here.