I usually rely on the “new posts” markings (orange icons) when I’m browsing on the forums. The FA server apparently keeps track of the last time I logged in and marks topics and posts that are newer than that.
Sometimes I want to check in briefly and maybe respond to a private message or a single post on one topic, but I don’t have time to go through all of them. Or I run out of time and can’t finish in one sitting. But then all the messages that had been marked as new, will then be marked as read the next time I log in, even though I never got to look through them.
So sometimes it would help to be able to select a date and time by hand, and use that instead of the time of my last login. Or to tell the server that this particular login is just a touch-and-go, and not to record it as my last real login. There might be other ways to implement it, like some “Easter Egg” trick or magic incantation in the URL.