Takes me back a long way to a chuckle.
We all were good mates - cockpit and cabin crew - and we listened in routinely for the sensible reasons described previously.
At one point the cabin conversation took, shall we say, an interesting turn. My mate up front, taking the initiative, started with a very slowly increasing volume ... heavy breathing ... into the intercom.
After a while the penny dropped and the purser exclaimed, in a loud voice, "(name deleted) ... (naughty word deleted) ... off". Chuckles all round.
Oh well, we didn't really want any more coffees that day, anyway.
Seriously, though, discreet monitoring can reduce the incidence of those situations where the purser hangs on to whatever might be the problem for a bit longer than might be useful. The advance notice via monitoring can make the flight management side of things a bit easier for the cockpit.