https://www.openstreetmap.org/user/StephaneP/diary/390290
If you have a very accurate GNSS receiver which shows you WGS84
coordinates, and place it on a survey mark, because this mark moves, you
won’t get the same coordinates year after year. If you surveyed a mark in
Australia 13 years ago, today’s coordinates are 1 meter off. So, accurate
WGS84 coordinates are not accurate at all if you don’t know the epoch, just
as a time value is unuseful if you don’t know the time zone. Working with
dynamic coordinates would be a nightmare for the surveyors. To make their
life easier they use a datum associated with their continental plate,
locked at a point in time. And for a few decades now, many countries have
chosen an ITRF datum and “locked” it at a specific date. With such a datum,
the coordinates of survey marks remain the same over the years.