Depending on the region and culture, the beginning of the day has been defined as the moment when the Sun rises or when it sets. The hours counted from sunrise are called “Babylonian hours“. They were in force among the Chaldeans, Egyptians and Greeks, in particular. The hours counted from sunset are called “Italic hours“. This was the practice among the Romans (hence the name), the Chinese, the Hebrews, for example.
It may seem natural to start the hours with sunrise. But this poses a problem: it will be “noon” around 4 in winter and around 6 in summer (in Paris, the Sun rises around 8:40 a.m. on December 21 and at 5:47 a.m. on June 21). Not very practical…
Italic hours may seem less intuitive. In fact, they have a practical advantage, as they allow people to know how much time is left until sunset, which will happen at the 24th hour. So, if it’s 8 p.m., the sun sets in 4 hours. We know how much time is left to do outdoor activities. And this is true whether it is summer or winter, since we have begun to count from the previous sunset and the length of the day varies little from one day to the next. In some countries, the backwards hours were even counted from 24 to 1 (instead of 1 to 24), which avoided subtracting 24.
But these methods still have a flaw: the day does not have a constant length depending on the season and the length between noon and sunrise or sunset is variable. Now, the only objectively observable and (relatively) stable thing is the passage of the Sun at the meridian of the place: true solar noon.
A good compromise is therefore to measure true solar noon and count the day from 12 hours earlier (midnight), so that you don’t have to change the date in the middle of the day. This is what the practice has been established on.