The statement by President Trump that the United States will designate Antifa as a terrorist group is an important one. The biggest reason is they are one, so we can now properly frame their actions. But there are other advantages both in preventing more attacks and catching and punishing the perpetrators.
Their actions fit the definition of terrorism as I pointed out in this piece calling for this designation:
“The president’s accurate description of Antifa fits the definition under federal law of a domestic terror group. Under that definition, such a group breaks laws ‘to intimidate or coerce a civilian population’ or to ‘influence the policy of a government by intimidation or coercion.’”