Think of scheduled interactions as calendar items. You add items to your calendar so that you don't forget them.
Think of interactions as meeting notes. You record meeting notes after an event takes place so that you have a record of what occurred.
If a scheduled interaction exists for an event, it is better to tie the interaction to the scheduled interaction.
But if the interaction wasn't scheduled in advance, you can still record "notes" for it. Just add an unscheduled interaction.