I believe it is because Oro and Sasuke really do have the same kind of bond Jiraiya and Naruto have. Its just that there messed up people and therefore they have a messed up bond. Whil Tsunade and Sakura's relation is more of a Teacher and Student type thing, Naruto and Jiraiya is more of a fatherly figure best friend kind of relationship and Oro Sasuke is more of a Heartless Indifferent sort of Relationship, but I really do think that Sasuke looks up to Oro as his sensie and Oro thinks of Sasuke as a Student.
With that said, back stories are the equivalent of the author realizing that the series is almost over and yet there is still much money to be made. So instead of just heading to the ending you will be hit with filler of Oro's back story, the reality of the shinobi, people laughing and glaring at each other... The filler is strong in this one.