The partition, and when anyone actually uses it

The commonest genuine use has nothing to do with the chauffeur’s discretion and everything to do with the passengers’ confidence in it. Two people discussing something sensitive on an hour-long journey will generally behave as though the front of the car is empty, and a physical barrier converts that assumption into a certainty for anybody who would rather not rely on assumptions.

Two colleagues talking about something they would rather not have overheard, on a journey long enough that behaviour alone feels insufficient. This is the commonest genuine use and it is entirely reasonable.

A well-behaved chauffeur is already effectively absent, and there is a difference between being confident of that and being certain of it. The glass provides the certainty.

Nothing in a car is free of consequences and a partition takes several things away in exchange for what it offers. Some of them are physical and one of them is social, and it is the social cost that explains why so few passengers use one even when it is fitted.

Headroom, occasionally. A sense of enclosure that some passengers dislike intensely. And it makes conversation with the chauffeur a two-step process, which is enough to stop most people using it at all.

It also means a passenger cannot see the road ahead, which a small number of people find genuinely uncomfortable.