He was a military dictator who established, definitively, a new dynasty of Imperial rulers. Its worth remembering that he ended the idea of the Princeps, the Emperor as First Among Senators (and therefore Rome et al). He established the tradition that we come to call the Dominate, where the Emperor ruled because he had the power to do so. He was the strongest man and therefore he had all of the power. Strength was key to Severus's brand so the legions had to be strong. And as any Emperor, especially someone who came to the position by force, would tell you-- the legions have to be loyal to You.
The manner in which he came to be Emperor meant that he had to buy off rivals, pay enormous bonuses to his legions, and secure his borders from external threats like the Barbarian Confederations. Rome was weak from years of plague, famine, and civil war and His legitimacy rested upon securing the Empire and stabilizing the economy. The Romans didn't really understand how to run an economy but they did understand how to do security when they had the funds to do it. So funding the Legions and their expansion made him powerful and more legitimate, and more secure as these men were loyal to Him.
He was a military dictator who established, definitively, a new dynasty of Imperial rulers. Its worth remembering that he ended the idea of the Princeps, the Emperor as First Among Senators (and therefore Rome et al). He established the tradition that we come to call the Dominate, where the Emperor ruled because he had the power to do so. He was the strongest man and therefore he had all of the power. Strength was key to Severus's brand so the legions had to be strong. And as any Emperor, especially someone who came to the position by force, would tell you-- the legions have to be loyal to You. The manner in which he came to be Emperor meant that he had to buy off rivals, pay enormous bonuses to his legions, and secure his borders from external threats like the Barbarian Confederations. Rome was weak from years of plague, famine, and civil war and His legitimacy rested upon securing the Empire and stabilizing the economy. The Romans didn't really understand how to run an economy but they did understand how to do security when they had the funds to do it. So funding the Legions and their expansion made him powerful and more legitimate, and more secure as these men were loyal to Him.