(I work at Quill). We've found threading to work really well with teams as small as 3. (2, probably not.) In professional contexts, conversations tend to be about specific topics -- someone posts a GitHub issue, bug report, feature discussion, etc, vs say a more social conversation that is less structured.

It's a way to keep track of the conversations you're having instead of a stream of consciousness (with multiple conversations interleaved) that is often times what people find in a channel. Once you shift to using threads, you sort of unlock a lot of things: revisiting old conversations via search, restarting conversations becomes trivial (all the context is right there!), managing your notifications (per conversation instead of per message now), quickly catching up on what you missed vs. sifting through hundreds of messages.

We're mostly solving for teams bigger than 3, but, we think it's really important that it does scale down to small teams.