You might not see the logic in it, and how it could be of any use for you, and that's okay.
But it could also help plenty of people out there.
The company I work for, we all work on inDesign but all on different versions (some even run on CS6 to avoid paying monthly), so we use IDML files anyway. Once we finished our part, we send the idml file to the person who will take care of comping everything, adding the final touches, and making the final project print ready.
In such instances, there WILL be changes made by the person taking care of the final steps. So knowing that, there would be no issue for us to have IDML files exported from Affinity Publisher. Some of us are even waiting for an idml export feature to switch to Affinity.
It's not because the export would not be perfect that it means it cannot be useful.