> who is still on the call

On internet / app based systems yes ... but on legacy telephone systems you have to remember all 16 of the '<Person> is joining the call' and mentally check them off when you get the '<Person> is leaving the call' on the way out. Of course you have no idea who joined the meeting before you arrived.

You didnt even have to make the mistake once to know not to keep talking on the call anyone can dial into after you think everyone left.