Showing 3 second average power is useful when you’re trying to keep to a certain power number. If it changes instantly you can find yourself trying to make small adjustments constantly.
But showing the power number and reporting the power number are two different things. As Andy says reporting power instantly will make it easier to react to a breakaway. That’s not the only time it’s useful though. It allows you to judge your own attacks better. It’ll be useful when you first hit a gradient, when you’re trying to maintain the same position in a pack, when you get dropped from the pack, etc etc etc.
Basically the sooner you can react to any situation the better.