User blog comment:Dragonjet/KC3改/@comment-73.158.62.98-20151122202558/@comment-1820055-20151122225936

It was added during the days when API links started to expire, where people were also reporting catbombs after iding. It was added to tell people how long they have been idle, to either (1) properly determine the minimum idle time to cat, or (2) once time has been determined, prevent users from getting cat by doing anything just to reset the idle time before it expires

After the issue, we intended to remove it but some users asked to let it stay, as so there we have it :)