I’ve had a Powertap P1S for about 18 months now. I’d say its definitely nice and useful data to have, not at all essential. Even riding indoors, you could easily go by speed based off a rear-wheel computer. That being said, I have found it super useful in race settings, on hill repeats, and riding into wind. Yes, you could use RPE. I guess power just makes it a bit more distinct and objective. Power meter was also free, so….