>>2584099 (lb) (Kek)
LB is put up for those who watch with their phones. (Phonefag)
If you click on a link that is on a page other than the current one, it will try to load the old page with the linked post and take a crazy amount of time (and data.)
So...the custom was made to indicate if a post was in a page (bread) other than the current one.
In this way a reader could decide, "Is clicking that link worth my time?" instead of screaming, - ah. You are knew. I won't use those words. Let's just say, "Reeeee."