yeah it stops the internals of the valve moving so the actual washer moves
Why you ask?
I have just cleaned up the tap, and put a new o-ring in it, but as I was finishing the circlip fell out!!! The screw on top of the tap has threaded, and so I can't remove the tap head. This means that the cover that goes over the nut that connects the tap spindle to the tap body doesn't come off fully, and so I can't get the circlip back in place. (slightly complicated to explain!! do you know what I mean??)
New tap valves - even easier - and nice fresh new tap workings to boot. Take old valve with you to merchant with the handles and covers, hand it to the oldest wisest owl there, grunt in his general direction and watch as he wafts off down the corridors of spells and fairies and returns with a nugget of purest gold.