probably to do exactly what they did. specifically, to barricade streets from people also diving dumptrucks and whatever other heavy vehicle they might get. Chances are it was requested by the secret service detail.
It's either that or an MRAP, and MRAPS don't exactly convince people they're the Kinder and Friendly New York Police™️
It kind of feels like he's banking on the police being supporters of his and is attempting to get them to lighten their security around the building. That would allow his protesters to pull another January 6th at the courthouse.
Basically why anyone has been voting Republican since the 90s when Newt Gingrich and Rush Limbaugh took the platform from "mostly bullshit with some outdated ideology mixed in" to "99.9% bullshit by volume".