I am not sure why? Maybe he feels you should insulate or something. If this on an uninsulated outside wall like a basement or something? I was hoping a mason or tile guy would have an answer for you. Anyway, Did you ask the builder why? He might have a good reason. I am not thinking of one.:confused:
There is no reason why just that when they plasterboard and skim finish you have a more regular surface to work on, you may find irregularities as you work along and the tiles will amplify any bumps as they wont lie flat.