Stryd enables you to understand the real-time effort regardless of terrain. Let's say your power number is 270W on the flat, 297 W on the 2% ascent and 267W on the 2% descent, and 240 W on the 10% decent. You will know that you spend roughly 10% more effort running on the 2% ascent segment, similar effort on the 2% descent, and much less effort on the 10% decent segment. Pace cannot tell you that, and HR can not unless it’s a super long hill as HR lags and is affected by many factors.
Stryd helps you prepare better for your race, and execute your plan with even pacing. How do people traditionally select a pace for the race? You might guess the pace you think you can sustain, print a pace band to tell you pace for each mile and try to guess how to compensate for hills on the course. How does it work with power? Well, test yourself on a short distance or use previous race results to understand your baseline. With this baseline, Stryd can predict the power you can maintain for an event to so that you can finish strong and optimize your performance. During the race, you only pay attention to one number: power!
With Stryd you can quantify running stress for the first time. To quantify stress you must use a metabolic estimate. Running 50 miles with 3000 feet climb this week, then running 70 miles with 1000 feet climb next week, then running 40 miles with 5000 feet climb next week. Before power, how do you quantify stress? Pace? That would be wrong. Distance? How do you compare 40 miles with 5000 feet with 50 miles 3000 feet climb? HR? That would be the closest to power but there are many runners that cannot get a reasonable HR reading from the equipment they wear. And HR still suffers from variability and drift even if the watch provides high-quality HR measurements.
If you have a coach, Stryd makes communication with your coach crystal clear. Your coach will be able to monitor your effort, not only from the perspective of zone distribution but also from the perspective of your own mechanical output. No need to ask any questions to have a very good idea on what you did. No need to check with you segment by segment to ask for your "RPE". If have HR data available, power + HR is everything your coach needs to estimate your "RPE". The longer the course, the more obvious it is going to be.
There are many articles written by coaches/runners on how they use Stryd to improve training. They are not necessarily elite runners, but they all benefit from running with power.