It is a very simple form of pipette. Glassmakers made these to use at the bench. You dip it in your bucket of water, fill it with water, and then put your thumb over the large end so that you can apply a stream of water,fairly accurately, where needed. The plastic "fairy liquid" bottle has rendered it almost extinct, but some people still use them. According to Eddie King who works with me and started off his glassblowing career at Whitefriars, they used to make them there.
I think it is interesting because while glass blowers often had to make a lot of their own tools, and still do, so far as I know, it is probably the only one that they made out of glass, for obvious reasons.
A piece of glassmaking history that is in danger of getting lost, I suspect.