Most Roman historians agree that the Marian Reforms played an enormous role in the establishment of the Roman Empire.
Wages, plunder, and plots of farmland to retire to became increasingly important to the average Roman soldier as the Roman army became increasingly made up of the poor. As a result, the loyalty of a Roman legion became much less about how much legal authority a general or official had and became much more about who was providing that legion with its wages, plunder, and plots of farmland.
Essentially, Roman legions became increasingly loyal to the generals who led them over the Roman state.
This loyalty to generals became problematic more than once in the years following the Marian Reforms, but when Julius Caesar took advantage of his soldiers’ loyalty to him, he managed to lead his men to victory in a civil war against the Roman government, effectively making himself the ruler of Rome.
Following Ceasar’s assassination, more civil wars were fought, ultimately resulting in Octavian (AKA Caesar Augustus, one of history’s greatest douc… nevermind) seizing power and establishing the empire.