The 688 bearing adapter is made of aluminum. The bearing adapter fits over your 688 mini bearings so they can be used in a 608 wheel hub and you need 1 adapter per wheel. There are two advantages to using mini bearings.
You create a much lighter setup as a 608 bearing weighs 12 grams compared to a mini bearing at 4 grams and an adapter at 1 gram
Mini bearings spool up faster than regular 608 bearings