Before the advent of Digital Modes on EME, nearly all QSOs were conducted
using CW. Occasionally SSB was also possible when one of the stations was a
Superstation.
Stations generally had large antennas and ran the maximum legal power, so the
potential for mutual interference was quite high, especially in regions like
Europe where EME stations were numerous and often located relatively close to
each other - less than 100km!
So a convention emerged with the aim of ensuring as far as possible that
closely located stations all transmitted at the same time, and received at the
same time.
The convention required that the station further to the east transmitted
in first period, and that further to the west in second.
Periods on 144MHz were usually 2 minutes in scheds, but 1 minute was normal in
contests, so the system was not perfect, but it certainly helped.
In practical terms for Australian stations, VK transmitted first for Europe near our moonset, and second for the USA at our moonrise.
With the advent of digital modes and smaller EME stations, the potential for
mutual interference might seem less, but there are now many more active
stations.
It seems to me that there is still much merit in continuing this convention with
the WSJT modes, even though many new stations are obviously unaware of the
tradition.
Of course there are times when it is not possible to conform.
Expedition stations to rare locations nominate their period in advance (usually
first period), regardless of who they are working.
A station calling CQ does not know whether an answer to his call will come from
east or west.
Nevertheless as a general rule for VK stations: