Trivia: The big fight scene is spoofed in "South Park" when the crippled kids Timmy and Jimmy fight, instead of the film's Nada and Frank. The episode is called "Cripple Fight".
Trivia: Notice that in the television station the SWAT team members use radios that look exactly like PKE meters from "Ghostbusters".
Trivia: Roddy Piper and Keith David decided to fight for real, only faking the punches to the face.
Trivia: The move that Roddy Piper uses to finish the fight against Keith David is one of his signature wrestling moves.
Trivia: According to Roddy Piper, he came up with (arguably) the film's most famous line. While driving one day, he scribbled various versions of the line on a pad, based on interviews he had done as a wrestler and gave them to John Carpenter. The line selected was from when Piper faced Playboy Buddy Rose, and said, "I'm here to chew bubble gum and kick ass." "And I'm all out of bubble gum" was added on. (Source: ScreenRant).
Trivia: The homeless "camp" seen in the movie is real. To save money, and for realism, director John Carpenter shot the scenes at an actual homeless encampment. The people living there were paid as extras. (Source: ScreenRant).
Answer: As part of the alien takeover, they're suppressing any resistance and discussion of the plot. They know there's a group, led by the preacher, who are onto the conspiracy and that they're somewhere in the camp. They destroy the camp to disburse them and prevent word from getting out.