From what I've seen and tried, tags like "ITUNESCATALOGID", "ITUNESARTISTID", and "ITUNESGENREID" do visually nothing on all my files. I wanted to know if it was worth adding these to my files at all.
This could also be because of the rebrand from iTunes to Apple Music, but I'm not 100% sure.
Any answers would be greatly appreciated!
These are fields primarily used within iTunes and Apple Music. If you don't use these players there is no other function they serve AFAIK.
I am not really sure what differences in appeareance or other behaviour you expect. Could you elaborate a little?
I had nothing particular in mind, I was just wondering if they did anything at all. But I've decided to not add anymore, since they haven't done anything, and it looks to me like it's a waste of time.
They help Apple to map a unique identifier to an also unique string.
E.g. the ITUNESCOUNTRYID would always be the same but would appear in the local versions of the player with the local name, e.g. in my GUI it would be "Deutsch", in an English GUI it would be "German".
The others help to reduce the amount of stored data: a number (id) requires less storage in many cases than the legible string. The string has to be saved only once in the database and can be referenced by the ID.
After a bit further testing, I've found out these tags only work on .m4a (AAC/ALAC) files.
Catalog (song) and album ID tags allow you to link right to the actual song or album on Apple Music respectively, making it much easier to copy and share the link. And the Advisory tag will show the explicit icon next to the song title. There might also be a bit more that I missed, but this is what I've found so far.
I was previously testing it on .mp3 files, which did nothing except add more data. 
