Whilst I understand the need for sercurity and that it is inconsitent all around Australia. This rule in BNE may be the guards covering their own a#@e. A few years ago in MEL an expensive necklace or braclet went missing and the gaurds were blamed, and I think charged, but were found not guilty (I could be wrong on this).
If you have an expensive watch, then if it goes in your bag then it won't be lying around at the other end of the machine for anyone to take. This is the only reason I could think of for their request.