IMO far and away the most valuable Plat benefit.

If all my changes and cancels had cost $50 per ticket, I would have incurred something like $500 in change/cancel fees so far this year.

And as others have commented, it's not just the money, it's about the award booking strategy. With free changes, I can book what's available now and then continually seek to upgrade the itinerary with better / more convenient flights. If I know changes are going to cost $50 each ($100 for a 2 pax itinerary), my entire approach to award booking gets revisited.