It was an interesting, and sad, flashback arc. The Norland history doesn't actually have any concrete meaning to the events occuring in Luffy's timeline, indeed, everything would make pretty much sense even if we had never learnt about Norland. However, it really makes a strong background story to the setting and many characters and the situation in general.
There was one thing that doesn't make much sense though: It was 100% obvious the king wanted to rob all the gold from Shandria, made even more evident by the fact he filled the ship with his soldiers. Yet Norland took that invasion force willingly to the island of his friend. Past actions had proven Norland wasn't gullible, so why did he agree to take the king there?
If he had refused to take the king to the island, they might have taken it as a sign the island didn't actually exist, but probably wouldn't have executed him for that.