The Difference Between Importing & Subscribing to a Calendar
When you import an .ics file, you get a snapshot of the events in the calendar at the time of import. Your calendar doesn't refresh the imported events automatically -- even if the calendar's owner makes an update. This is a good way to add events to your existing calendar that aren't going to change, like tide tables or phases of the moon.
Subscribing to an online calendar connects it to your calendar. Whenever the owner of a calendar you've subscribed to makes changes to the events, your subscribed calendar updates itself. This is a good way to track events that change frequently, like movie times or a school calendar.