The last line of defence against accidental WWIII

[bitchstewie]

Trucks Get Parked Over Air Force Nuclear Missile Silos During Tests… Just In Case

[LimaDelta]

The article comes close to, but doesn't fully explain the main reason - that is these silos are at known locations and are watched. The truck just indicates to any observers that despite all of the actions being taken building up to a launch, it is for training and testing purposes only. Without this obvious signal, those actions could be misinterpreted as an actual launch. It's not there to physically stop the missile, per se.