I also assume it's a watch pocket. Like @dieworkwearI have a couple of pairs of bespoke trousers with it present, until I noticed and put a stop to that. It's useless. Furthermore, unless the trouser maker is paying attention, there's a good chance that he will sew suspender buttons (if you've asked for such) to the inside of the pocket and not to the actual waistband on the trousers, thus rendering the suspender button gravely compromised in its job.
I have eliminated all such things in my trousers now I know to look out for them. No belt loops, no watch or coin pockets, no "pockets within a pocket", and no rear pockets, either.