It's a tactic they've borrowed (adopted?) when they integrated a lot of Neo-Nazis. Moving vans and panel vans like this, unlike something like an APC or a marked vehicle, are relatively inconspicuous, relatively common, and more importantly, give no indication of what's in the back, or how much. You could park and dump 2 guys and landscaping supplies off, or you could park and 10-20 guys in full battle-rattle could pop out. It's a tactic of terror - "oh shit those moving vans have been an ICE thing, is that van at Home Depot gonna disgorge a pile of guys with guns?"

More importantly, they let you disappear. Nazis like them because you could IN THEORY commit something like an infrastructure attack or even a mass shooting like some guys in Idaho tried to do at Pride last year, pile into the back of the van as you finish, and drive off without many ties back to who was driving the vehicle or what or who was in it.