It's an ancient trope and the source of many amusing cartoons, newspaper comics, and sight gags in old movies. I don't know the exact origin. But the idea was that if someone was insane or was an escaped mental patient the men in white lab coats would show up with butterfly nets to catch them. Once they had the crazy person trapped they usually injected tranquilizers using a comically large syringe, strapped them into a straight jacket, and hauled them away to an insane asylum or sanitarium. Don't try to make it make sense, just enjoy it for its retro weirdness.

Edited for better wording