Marking posts as read and hiding read items from your feed is a handy way to keep your feed free of clutter, however sometimes these features are too powerful.
Commonly, in other apps with these features (I have not gotten hiding read posts to work with Android/Piefed), I've had the problem where I read a post, back out, then decide I want to intact (comment, share, upvote), but the post is already gone.
Being able to differentiate "recently read" posts (those read, say, <5 min ago, and/or since the last app refresh) from the rest of your read posts would then allow for custom actions, like not hiding recently read posts.
An example of an app that uses this feature well is Feeder (the FOSS RSS feed reader), which can hide read articles but still allows you to scroll back a bit to grab something that caught your eye.