I think it would help with fault-finding.
I think it would help when trying to describe the wiring arrangement to another person.
I think it would help to get some kind of 'consistency' when wiring up a lot of intermediate switches, in the lottery where some electricians like to wire things up first time to every switch can be in 'off' position when the light is off.
It would, for example, be very difficult to talk to a poster on DIYnot about an intermediate switch without being able to identify the terminals. (The top right terminal. My right? No your right. Oh bugger this.)
That said, some manufacturers make little or no attempt to make the intermediate markings correspond in any way to the two way switches.