In triathlons, time trials and mountain bike races it is crucial to pace yourself accurately from the start. There's no pack to sit in and recover from early mistakes. The power meter is a huge advantage at the start of these types of events where you can have a cap set on the watts to ensure you start at a pace you can maintain for the duration.
In long distance events, such as ultra mountain bike races and Ironman triathlons, all of the pacing mistakes are in the first two hours and those mistakes are paid for many times over in the final two hours of the race. I'll bet you've never, ever heard anybody say, "I just went too fast in those final two hours." I'm sure everybody has gone out too fast at some time in some race. Unless you're ignoring your data, a power meter will prevent you from being this dumb ever again.