iPad offers two formats for photos, HEIC and JPEG. If we import into Photos and export from there, the format (HEIC or JPEG) is preserved. But if we "Edit in GraphicConverter," GraphicConverter gets TIFF.
HEIC and JPEG are both lossy formats. Which means, each time you change anything and save the file, the image looses in quality.
Photos and your iDevice usually avoid this by adding all edits as a separate file and leaving the original untouched. However, third party software would not understand this additional information (especially as it's an internal format and can change with any update of Photos or iOS). So Photos reverts to the old fashioned approach to avoid image quality losses with every edit: saving the file (with all the edits you did on the phone or in Photos) in a lossless format, TIFF. This can be edited as much as you like, and will not deteriorate each time you save it. At the cost of being more bulky.
Once you are finished editing, I suggest making a JPEG or HEIC copy and keeping the TIFF file only on your backup disk or such.