You want them all in one place so that they can be uploaded in one go to your fitness tracking site. That lets you see what your HR, power and cadence all where on that specific hill for example.

I used to have a Fitbit that used my phone for GPS and it was awful and drained the batteries on both devices. I guess the idea of having 5G is that you don’t actually take your phone with you, one less thing to have to force into your pockets.