It allowed you to post a larger image which could be used to highlight detail better.
It took strain off the FB servers.
It allowed people who aren't on FB to view the pictures.
It allowed me to post the same pic on multiple pages without having to upload it to each.
It allows me to delete a picture if I no longer want it posted (inability to edit posts on FB would mean uploaded pics are up indefinitely).
There were perks to it and those who used it a lot had their reasons. Those who think it was foolish had theirs too. In the end Photobucket screwed over a lot of people.
-Tim