Excellent photos Rob and fabulous rebuild, well done to the RFAM for this aspect of care for the historic site.

A watch office with a visual position was important before days of radio for the reason of looking out and making sure the the field was clear etc, think FISO but without the radio.
As mentioned above, alerting ground services was an important role.
The visual position was also useful for duty pilots to keep their eye on flying training and activity in general on the airfield.

Exactly what could be done to warn pilots of any possible problems or obstructions that may hinder them I don’t know, maybe coloured flags, or vary pistols were in use then?
There was also a system of markings on the ground and displayed on masts etc, but unsure if they were in use that far back.

The AM/RAF certainly used the term Watch Office into the post-war period, Control Tower seems to have come more from the civil side of things when their structures were definitely becoming more tower like, Croydon being a good example.