My guess is that it was used for passing messages/paperwork up to the cockpit window/dv panel when the engines were running and the doors had been closed.
Likewise, completed paperwork like tech log pages and load sheets could be attached and passed back down to the ground crew.
The other possibility is that it was used for passing messages to crew members situated behind the mainspar.
One of my old squadrons was equipped with the first Blenheim IVs. There was no intercom so they came up with a pulley system with twine and a bulldog clip so that the captain could communicate with the air gunner who sat in glorious isolation behind the spar.