When the romans created a 100% profissional army was with Gaius Marius in the 2 century BC, it was necessary to boost the army by recruting in the jobless and poor people of Italy, and it helped shape the Roman army and then they expanded alot in territory, later when the mercenary forces started fighting along side the romans 400 years later it was more by desperation because they needed help to fight Atila and the Huns.
Thank you for the comment @clearshado