Well done Thomas, yes it's to signal how much of the anchor cable is still out, he'll signal as the relevant markers appear in the hawse pipe.
Although the Officer on the platform is in communications with the bridge by radio, ringing the bell lets everyone working on the FX what the state of the anchor recovery is.
When i was on Ocean Village, we were anchored off Villefranche-sur-mar for a days call when, as the anchor was being recovered the anchor clutch motor burnt out. The only answer was to cut the anchor cable and drop the anchor to the sea bed!
We sailed around for about a month with only one anchor until, on a turnaround day in Palma, we had a new replacement anchor delivered and, after a very long day for the deckies and tech team, it was fitted and we sailed only slightly late.
Not the easiest of jobs fitting a 12 ton anchor while alongside!