In short, Trump ordered violence by others in order to create an illusion of his own strength and courage.
Of course the Secret Service often have to be rough in order to protect the president. But just as the violence of war is an argument against going to war lightly, the violence tonight was a good argument against the president doing frivolous photo ops in serious times. If Trump knew he was going to do this, he could have had the Secret Service set up the barricade further out before the evening protests got crowded. Then there would have been no shoving, no tear gas needed.
Ordering force against peaceful protestors always has downsides. It physically endangers the protestors being shoved and tear-gassed. It physically endangers the police who are thrust into altercations. It also has a moral cost. At the root of the tension behind the protests is a frayed relationship between law enforcement and the public. The attacks on peaceful protestors and journalists Monday night will make that worse.