> What is the usefulness of this information?

My guess is that it's there for the sizable population of facebook users who check it compulsively. For a lot of people, the appearance of constant activity is going to be a lot more interesting than the appearance that nothing is happening, even if for only fifteen or twenty minutes. I can see this increasing 'engagement' a lot, and it's easy enough to ignore for people who find it annoying.

And your optimal signal-to-noise ratio is still there in your news feed.