Skip-the-line tickets are quite often the reason you sign up for the tour in the first place. Except for the Eiffel Tower (where they also have their own rules and only people with "almost-impossible-to-get-pre-booked-on-internet-time-slots" can enter with just the security line), no professional guide (or appearing to be so)  will make you stand in a line because it's quite easy to skip a line when you know the tricks and even easier when you have an official guide who automatically has the proper credentials!