I agree with
@AliceShiki that it's handy for people who have a hard time remembering names. And even for other people, I think it might depend on the names a bit. Like, some people have trouble remembering names of specific languages (I have a very hard time with Korean names, for example, and sometimes jumble up Japanese ones) or can't deal with fantasy names very well.
Furthermore, the number of characters and the length of a novel can, IMO, also influence whether a glossary might be needed. Like, the more chapters (or rather the amount of words) and the more characters there are, the harder it might be to remember all of them. (Not for everyone, obviously, but at least for some people.)
Like, personally, I did start a glossary (even though I didn't update in ages ...) and while I doubt anybody will have a problem remembering the names of the main characters themselves, there are dozens of supporting characters that will sometimes only appear once or twice per volume and then maybe not turn up for a few at all.
I absolutely do not expect my readers to remember every single one of them. Of course, when they do turn up after a couple hundred chapters again, I will put a short reminder in the text as well but even with daily updates, a chapter 400 chapters ago would mean that a year has passed since the character appeared for the last time and not everybody will know exactly who is meant even with that and rather than pondering on it, looking it up in a glossary is easier.
Other than that, I've noticed that some readers have more trouble remembering the relationships than the characters themselves. Like, I have the kind of novel where somebody will reincarnate for a trial, have a wife and child, get killed, return to his own life only for the child to later turn up as the fiance of some girl working for the mother of the reincarnated main character who is - in turn - the child's cousin
I guess it's unsurprising that it can get confusing at times.
Unfortunately, SH's glossary isn't really good for showing that. Like, sure, I can write down who is the child of whom and stuff but it's just not intuitive. I recently made my own relationship charts to solve that issue and feel like it turned out quite well. I need a bunch though since it's just way too many characters to put it all in one. It's kind of crazy how much actually happens over the span of a few volumes ...
Anyway, TL;DR: I think there are valid reasons for having a glossary. Besides the readers' ability to remember and the memorability of the names, the time since a character was last mentioned, and a character's role in the story can all influence that.