As other people have commented, it's partly a relic of an era where things were typeset by hand. Some journals may also require this in their style guide; I don't know of any offhand but the list of journals is long and their choice of style is varied.

> I dunno, do people actually read through a citation list, in a manner where the redundancy would be relevant?

Yes, they do. When you work on a Ph.D.-level dissertation, you end up having to learn your subfield pretty well: its major works, open questions, current major players, their ongoing arguments, etc. You start to notice citations cluster, and you can often find more than one opinion group. People you have to listen to (ie: your advisor / committee) will hand you articles with authors you don't know and who may come from labs that you don't know. One common way to start orienting yourself is to look at who they cite.

You pick up patterns: some people love to cite themselves and their friends, some people really try to give a detailed overview of the state of the field, and some people go out of their way to attack their rivals via citation. Sometimes the reference list has an author or work you don't recognize, and might have missed while glancing over the text. Mostly it's something mildly relevant and occasionally it's an exciting direction to look into.

The really long citation lists, where the triple emdash would make sense, mostly occur in fields outside science and engineering where knowledge is exchanged through chapters and books. Some fields expect a lot of detailed textual analysis which can involve a lot of careful citation of the same chapter or book over and over again.

While I've never worked in these fields, I've been friends and roommates with people in them, and I quickly learned that bibliographies are way more of a thing for them than I ever imagined.