Right, but if your client immediately kills a session, and that client also needed new cookies, maybe that client was incognito. If the session stays open longer, maybe it was a legit new user.

isn't it a little late to do anything with that knowledge if the user is already gone from your site?

if they posted something and they're a bot, then you want to take that stuff down at some point.

But there's a measurable difference in the browser's behavior between the two modes, so you can determine which one is being used.