It’s just in response to “it's not always clear when the job is done”. With shell integration installed, iTerm knows how to separate commands and responses (you can cmd-shift-arrow to move between them, select the entire response with a click, etc.) so I thought it might expose that knowledge. It does that just by putting a distinctive character sequence in the shell prompt, and you could do that directly if needed. Other REPLs have customizable prompts too.
I wonder how much of what iTerm knows about the current terminal state is exposed. I got it working "good enough for me" and moved on to other pieces of the puzzle. I'm sure what I did could be improved upon quite a bit. I bet you're right that there's additional juice to squeeze out of whatever iTerm exposes.
