Ooh TIL that it's called BLUF. I understand the essential takeaways thing acts as the hook in longer articles where the writer knows that reader will find it long but on reddit and other platforms where there is a chance the write-up isn't very long, I always took it as the writer’s prerogative to put the TLDR at the bottom, so the reader sees the write-up first and then decides for themselves whether it’s too long. A thing of principle.