Quote:...I am a casual user, usually just removing an odd frame, blurring the background or layers to create thumbnails or backgrounds for videos, which means opening a file, adding some brushes, using clone brushes and retouch and saving it. That's all...
It is not that much different from PSP. The native Gimp save as in File -> Save is Gimp .xcf
Same as PSP .pspimage (or whatever they are calling it these day, long time since I used PSP) that saves all layers / masks / channels / guides / active selections. It is meant to prevent users inadvertently losing information from a format, jpeg or png which is single layer, flattened.
If you want any other format than .xcf just go to File -> Export or File -> Export -As and enter a filename with an extension say my-new-icon.jpg and it gets exported as a jpg.
Note that in Edit -> Preferences -> Image Import & Export there is a "Export File Type" setting, default is png but you can change to any of the common formats. Can save a tiny bit of typing.
and
If you open a file-type.xxx , edit it , and want to overwrite, there is the option in the File menu, appears just above the Export options.