The app uses it's own specific working file format (.afphoto in this case) as default for "Save" and "Save As ...", which keeps possible applied settings, changes made, added or modified layers etc. in a more reproducable manner. All in all similar like Photoshop, which uses .psd as it's main file format. - Further, if you already have an existing TGA file, what would you gain from saving over the same exact image you still already have on disc without applying any manipulation (?).
However, for Affinity apps one always has to export to other file formats, they don't treat the file extension, or a loaded in file's binary format contents as an file type indicator for their general file saving operations.
