What is this frame feature for? I have a Kinesis Tripster ATR.  The drive-side seat stay has a lug on it whose purpose I can't work out:



There's no such feature on the other seat stay.  What is it for?
Its a chain hanger.  Purpose is to hold the chain when you take the wheel off. 

The idea is to support the chain and stop it falling through itself, making a tight loop and shortening the life of those links.

They're not common on carbon bikes any more, but handy on any working bike.

Don't forget to undo the chain after reinstalling the wheel and before riding away.