Stick it in the spout of a teapot and it stops the tea leaves (and calcium) coming out. Tracked down from an initial sighting at St John Bread & Wine and available from a rather good new outfit called Jenier Teas www.jenierteas.com
Loose leaf tea is not found very often - teabag tea is just so much less mess - but it carries a huge quality premium and cachet.
See post http://greatcafes.blogspot.com/2011/08/tea-pot-spouts-strainers.html