Why did men join the Roman legions?

Because they had to be Roman citizens to join and the men needed to be over the age of 20 so that they could join

In order to become a Roman soldier the men needed to over the age of 20 so that they could join one of the Roman Army legions. The main Roman soldiers were called legionaries and they had to be Roman citizens to join ... Other soldiers were allowed to join the Roman Army and were known as the auxilia