The reason the federal troops were there was because the police in Portland were not protecting federal property. The police in Portland are capable of protecting that property, but they were not doing so. A genuine answer to your question is that there is nothing that the feds were supposed to do that regular police couldn't - the regular police could, but weren't.
The idiot sending his goon squad there at all is why he's the most racist president since Woodrow Wilson.
There are already Federal Police that are allowed to work with local or state that are empowered to protect federal property. Standard service rifle, baton, tazer, and other such equipment, their own vehicles to take people down to whatever local place can hold them. And yes, ID and shield. If some buffoon runs up looking to attack them or property, they can use appropriate force. some guy setting fire to trash in the middle of the street does *not* mean it's time to assemble the Avengers. It doesn't mean you need an MRAP with a dude with a high powered rifle to threaten the crowd with.
You need a fire extinguisher (even in Portland, the protestors were putting out fires themselves), and *maybe* to arrest that one guy. In short, stop acting like you're in Fallujah fighting the Taliban - you aren't, and even if you *were*, the army knows better than to blast the girl with the flower on the news crew directly in the face with an explosive. Calm the hell down, if you leave the hot-dog seller and, the guy playing the drum, you'll probably get a hot dog, some water...and possibly a minor headache.
(And, as a hint, if some car comes speeding through shooting all of a sudden, it's probably a white supremacist, not BLM or the mythical ANTIFA. You know, the guys with the *actual* centuries-long history of murder? The guys that openly talk about how they want to show up at events and kill cops? More likely than BLM, with no known murders or attempted murders tied to it, or ANTIFA which, yet again, just means they fight against fascists?)
This goes for most interactions.
If that cop that shot Levar Jones,
or Philando Castile, had just been calm, everyone would likely be perfectly fine now. Yes, there are a *few* guys who may go all Quickdraw out of nowhere. But the cop is the professional, and the one that initiates the encounter. They don't need to lose their minds in fear if he does something they don't expect, or move too fast. Or move too slow.