Zulip has a thread based Instant messaging model which helps facilitate multiple different conversations in the same channel without losing track / context.
What is the advantage of threads over just more specific top level channels. e.g. in their example why not #annual-summit-name-tags etc. One immediate thing slack does wrong is it makes channels too hard to make and destroy and leave and join, but if that was fixed... What is the advantage of the nesting?
zulip ends up being a bit like e-mail where you can add people to threads and they immediately have the history. I've used mattermost, not slack, but that basic model fixes a lot of issues I have with mattermost.
A more specific slack channel (and I assume mattermost channel) is a thread that you can add people to any time and they will immediately have history. Or put in the inverse, a Zulip thread that is too general is a channel with information overload and becomes useless. Does the nesting have intrinsic value over simply more top level channels?
Right, in my view it's important that every message have a topic/thread. The reason is that you spend most of your time/effort in a busy team chat tool consuming other people's messages, and that flow is a lot more efficient if you don't have a giant "catch-all" bucket of unthreaded messages about fundamentally different things to wade through in order to find what's important to you.
Zulip, used well, ensures that the messages are partitioned into different conversations, so you can easily do things like skip to the bottom of a long thread to see if it ends in a satisfactory resolution, without missing the unrelated question that you need to answer.