An agent like claude might run a subagent (e.g. to fetch and summarise a web page, or explore a code question) as if it's a tool, so you can't really see what's going on inside it. Juggler makes these sub-threads part of the overall structure, so you can navigate into them.

Another really elegant thing that pops out of subthreads is that to do a compaction, you simply move the entire conversation into a subthread, and let the subthread summarise itself (which they do anyway). So we get compaction as part of the architecture, and you can also dig into that old thread if you need to revisit any of it