What is "PersistedContent" good for, if I can't access the properties of it's content?

Thy can be used in native activities,  You can download them for use in things like run, walk, and cycle.

With wayponts, if I recall, in CIQ you can get the waypoint names, but not their location.

what might be a meaningfull usecase? what might be a other solution to get waypoints on the watch, to utilize their coordinates for different calculations?

Again, they can be used in native activities/  Look at something like "run" and the options for course and navigation.

I first looked at getting the location of native waypoints about a decade ago, and in my CIQ apps. I save my own waypoints so I can do things like show them on a map, calculate distance, etc.

yep - but in this case, what might be a comfortable way to enter waypoint names and coordinates?

App Settings, and save them as data for your app.

On newer devices if there's a textual input then it offers to use the keyboard in the Garmin Connect phone app. I'm not sure if there's a way to activate this remote keyboard from CIQ apps, but it might be possible