On Wed, 22 Aug 2012 11:49:36 +0100 (BST), "Dave Liquorice"
wrote:

You could just open the tap. But some Bib taps have a built in double
check valve this could be to drain the bit between the two valves.

This is what I believe it to be: to drain the non return valve.

I think it is preferable to use a tap without a valve and install a
check valve inside the house to minimise the chances of it freezing?
At least, that's what I have done. The taps without an integral check
valve (ime, ymmv, etc) do not have this drain nut.