Not long ago the Apple Watch was very limited in what it could render on-device. So a bunch of apps had to do it this way.
That said, this is certainly a "special relationship" with big partners like Uber to get their apps working at Apple Watch launch in 2015[1].
That the access "wasn't removed already if it was no longer required", well... I think we can all understand legacy code.
[1] "It is likely that the Cupertino tech giant had to give this access after it gave app developers a four-month window to develop apps for its Watch before the unveiling of the product."