I remember being a kid and there still being commentators at Mass explaining what was happening following the wake of the changes that occurred the Second Vatican Council. The commentator explained what was going on and what the people were supposed to be doing. It seemed that, just about the time I was aware of it, there was a call to bring this role as it pertained to teaching people the Mass changes to and end.
Interestingly enough, this role, if truly needed in a given situation, is still permissible according to paragraph 103 b. The use of such a role is to be highly restrained however. There are a lot of phrases such as “if appropriate,” “briefly explains,” “thoroughly prepared,” “notable for their restraint.” In other words, it is not to be a distraction, but if really needed, to be an aid to lead people into the Mass.
The person may not give the instructions from the ambo but another suitable place within site of the people. It reminds me a little bit like a good commentator for golf: quite, reserved, and somewhat minimal.
When might one use such a person today? I was wondering about this myself. It might not be a bad idea at large weddings and funerals and other such events at which there may be many people in attendance that are not Catholic. It might aid them in understanding what is happening or at least what it is that they are supposed (or nor supposed) to do.