"Il faut comprendre les règles pour jouer."
Translation:You must understand the rules to play.
"you must" = "Tu dois/Vous devez" it's an order form for a specific person in French. While "it's necessary" =" il faut" it's an impersonal expression. So it's not more common. We just use it when in some cases, when we need it.
(is my explanation clear? Sorry for my bad English.)