I often borrow CDs from the library and add those to MusicBrainz. These will sometimes not include the backcover—and sometimes not even the booklet. This can leave only the medium itself, which will have a huge library microchip and/or barcode sticker on it, covering a lot of the print of the disc. At this point, the ability to read label information, cat. no., etc. off the release are highly diminished. I still have a 100% valid and official release, but there will be huge gaps in what I’m able to add.
Additionally, there are digital releases (e.g., on Bandcamp) where it’s unclear whether it’s released under a label or self-published, and even where it’s clear that it’s under a label, it might not be possible to say if there is a cat. no. or not.
As I write on https://wiki.musicbrainz.org/User:Freso/Editing_principles : “Missing data is better than wrong data, or “if in doubt, leave it out””. It is always preferable to leave out data to entering data based on wild guesses.