Jungle is the only role that is completely dependent on one summoner ability. You just flat out can't jungle without smite because you can't get the jungle items that make it possible. There needs to be a choice in the client to have a summoner skill auto set to smite when you are assigned as a jungler. The biggest thing is if a jungler forgets smite it ruins the game for 9 people because the game is no longer competitive.
Smite summoner spell is automatically assigned when you are jungler and playing in draft mode. Plus, this is not the subject of this thread.