For the same reason that anything is named.... so that when people speak about it, they are sure that they are speaking about the same thing.
Names are a form of code... once they are universally accepted, they save people going into great details when they wish to speak of something.
eras and periods
because it was different time periods that is why they did not live in the same era
Periods are subdivisions of geological eras.
Eras are divided into periods.
periods make up eras
Geological eras can be divided into periods, epochs and ages.
Geological eras can be divided into periods, epochs and ages.
periods
Most scientists agree that there are 3 eras for every eon.
Periods mean eras centuries ages
Eons, Eras,Epochs, and period
a