Chat channels give immediate and interactive feedback. When someone asks a question in Reactiflux, they frequently get an answer within minutes, if not seconds, and we can iterate on asking for details quickly if needed.
The downside is that multiple simultaneous conversations in the same channels quickly become hard to handle, questions can get missed if they're not answered right away, and we do see a lot of the same questions over and over.
I know the Spectrum folks have emphasized the searchability of their site as a major selling point, both from Google indexing pages and the ability to search for existing threads. I won't say they're completely wrong, but at the same time, my experience is that newbies in particular won't actually search for existing answers - they just want to ask and have someone answer directly.
I used to routinely go back through the day's discussions in #general and #need-help whenever I got home from work, and yes, reply to questions that had been asked 8-10 hours earlier. But, Reactiflux has become too high-volume for me to be able to do that.
That said, I still will scroll back in some of the lower-volume channels to see what came up. So yeah, having chat persistence is an incredibly nice feature, and I can't imagine doing without it.
[0] https://www.reactiflux.com