Originally people serving in the Legions were younger sons of land owners because they had to bring their own armor and were paid no salary, but got a share in the loot. There were enough of them at first to fill the need.
About a generation before Julius Caesar, it was changed so legionaires drew salaries and their equipment was provided by the state. Poor men could now join the legions and get out of poverty and they did! To keep the legions busy in times of peace, they were put to building roads and aqueducts and other public works. Landed men were mostly officers at this point. They were also able to justify a standing army since they were kept busy.
Julius Caesar elevated some hundred Legion officers to the Senate and it suddenly became a way of getting rank there. Augustus, I think, was the one who offered Roman citizenship and all the privileges that came with for military service. Later emperors extended that to people of other nationalities that served in the legions.
The key to Rome's success was the trade that flourished in the empire. In order to keep that trade going in an orderly manner, they had to have peace. The legions enforced the peace and expanded the empire to bring in more trade and more trade markets.
