As I understand it, the purpose of the selection feature is to highlight certain areas in a photo and add meta data or comments to that specific area.
An addidtional benefit of the selection feature could be that it allows for cropping photos/scans to the relevant content (instead of preprocessing these photos/scans in Photoshop or similar).
In this use case, the resulting selection should be shown in the main pane of the project view, replacing the original photo. A command like “Use selection as main image” in the selection context menu would allow the user to control this.