Personally I don’t see it. If everybody is going to see your underpants, I don’t see why the fact that they can see an outer pair of a number of pairs rather than just the one pair you’d be wearing normally really makes things better. But that’s what she did.
Anyway, I think the scene might be of interest because it would perhaps give some idea of just how detectable the wearing of multiple pairs of knickers would be to the observer. Would a teacher notice or not? I’d never noticed any sign of it until I heard the commentary. I’m not sure if I still do.