Trigger_once only works once, for certain events you don't want to be repeated. Like a falling boulder. You don't want it to be triggered again so it will climb up again, do you?
Trigger_multiple is exectly like trigger_once only that is can be oporated as many times as you like.
Here is a nice way for advanced mappers to control doors using trigger_multiple:
Stay-Open Doors By SlayerA