To the best of my knowledge, the convention is based on history. In previous decades, neo-pronouns like xe were proposed to serve as gender-neutral alternatives to he and she, and since they were new coinages, they didn't have commonly known objective and possessive forms, so all three forms where listed.

The pattern was so established that it carried over to he, she and they even though their declined forms are commonly known.

I think some of it is sort of to make people with more than one pronoun, or neopronouns, stand out less or feel less like an "othered" group.

If people with """""normal""""" pronoun usage just list their pronoun as "she" and anyone who uses multiple has to list "she/they" or someone who uses neopronouns needs to list "ve/vir/virs", this can make those parties feel more "other", perhaps. If everyone is listing multiple pronouns, even if they're sort of repeating themselves, then it levels the playing field and makes things more inclusive. Just a thought.

The other thing that comes to mind is that some people might be cool with "she" but not with "her". By specifying "she/her" you're explicitly telling people you're cool with it all.

At the end of the day the extra 4 or 5 characters isn't particularly high effort or high cost, and it seems to be what people run with for whatever reason they do - so I guess that's just how it be!

idk about the actual reason, but it's easier for me to recognise that someone is specifying pronouns when it's 'she/her' instead of just 'she'

I've always wondered the same thing. I think it's just convention at this point.

To me, it's an extension of spoken language. It'd sound a bit off and maybe actually have different connotation to say "I use he" instead of "I use he/him".

So yeah, I think it's to reduce confusion because outside of presenting your pronouns, two pronouns are unlikely?

I seem to remember years ago some people listing their pronouns by she/her/hers. I think she/her, she/they, he/him, etc. is just a shortening of listing all three while also being inclusive. As you mentioned some individuals are more fluid and may go by she and they but prefer she so it is listed as she/they.

It's redundant, that's just how people do it.

All this time I thought it would make learning the language easier for a non-native speaker, because it's reinforcing which pronouns go together—her with she, him with he, they with them, etc. Funny I never saw it as being redundant until you pointed it out.

It's weird to me too. It seems like a lot of people need two pronouns to register that someone is starting their pronouns, and it doesn't especially matter if it's two pronouns from the same set or two pronouns that represent different sets.

Sometimes, especially in the case of neopronouns, two set of pronouns share nominative form, but are distinguished from each other in their accusative form

For example: ze/hir vs ze/zir.

So, in those case, is to especify Which specific "ze" pronoun the person is talking about.

This also applies to "she" and "he" as pronoun sets like she/shim, she/sher or he/her are also a thing.