For me, this is fine for casual rides, and I have done this frequently. But as soon as riding becomes training, better understanding of your anaerobic and aerobic use, calorie and sweat estimates, etc, is super helpful for progress.

It's very easy to overdo it otherwise, or not eat enough. For instance, I burned nearly 2K calories in 3 hours of ride time yesterday. And when Garmin has access to power, cadence, and heart rate data, those estimates are pretty darn accurate since calories are energy.