I don't think most people need to read everything they receive, but in my role as the project leader of a large open source community, essentially every conversation in chat.zulip.org is potentially relevant for me, so skimming every conversation is useful.
The critical thing here, both in my daily work and coming back from time away, is that Zulip makes skimming really cheap (`n` to jump to next topic, then read a moment and then hit `End` if it seems likely that the thread is resolved to check the conclusion, and then repeat). So a 150-message thread debugging something takes like 30 seconds -- read the first post (maybe a bug report), note who was helping investigate, jump to send and see that the topic ends with a PR link or other resolution.
At the same time, Zulip's organization means I could find the dozen of threads that might be a short thread presenting a question or problem that I'm our main expert on. In these cases, I can read the thread carefully, and send a reply resurrecting the thread (2 months later) with answer that was missing, some added context/background, or a link to a PR I made to put the answer in our documentation (my preferred solution to unanswered questions).
This has the effect of unblocking a bunch of useful work that had been waiting for me to return, and also giving me good context on everything I missed.
It's also pretty fun to see all the great that folks did while I was away :)