Paul Hyett wrote:
Just one question : If hPa is the same as millibars
(as seems to be the case), why have two names for it?

They are not the same, they just appear to be because 1 hPa = 1
millibar. The bar is one of the old units for pressure. The pascal is
the SI unit.


It's like Celcius & Centigrade.

Those were two names for the same thing. The word Centigrade was
officially dropped in 1948. The correct name is Celsius.


As with general conversion to the metric system, transition takes time
and effort. The hPa replaced the millibar as the legal unit of
pressure for UK aviation in 1996, but the hPa value must still be
described as 'millibars' in spoken communication between ground and
air.


Lots of stuff about units at the official SI website:
www1.bipm.org/en/si/derived_units/2-2-2.html