I find power meters very useful both indoors and outdoors. But I find it a lot easier to average a higher wattage indoors simply because there is little to no coasting involved on the trainer. All my local outdoor rides consist of undulating rolling hills with very little consistent flat riding. So while I can get my indoor Variability Index (VI) very close to 1, outdoors my VI is more like 1.3 or even higher. So one thing I try to work on outdoors is applying more power on the slight downhill sections where it is so tempting to soft pedal, especially with a tailwind. A power meter helps me to see when I'm slacking! Power also helps me to pace climbs on long events. I spend the first hour trying not to over-reach my target power and the last hour desperately trying to maintain it! So works both ways, lol.