Precision as in it's easier to see the smaller increments of measurable difference in one unit of psi than it is with one unit in bar. Units are not regardless in the least. The overarching point is to match the gauge's measuring range with that of your current operating environment.
For reasons stated above, and more, oil pressure is very useful to monitor for someone well aware of its implications. Your lack of this belief doesn't negate its purposeful use.
Bars, centibars, millibars... The Defi unit displays Bars and centibars, which is more than adequate for this application. Match the measuring range with the operating environment, but don't change units (from bar to PSI, say) - that's just silly.
And belief doesn't come into it. As I clearly mentioned, I understand the usefulness of oil pressure monitoring generally, but it makes little difference on the Golf that I am at 4.1 bar yesterday and 4.2 bar today.