I have 8 of the 9 pieces of the original Treasure Map Pieces. What does it do? Is it worth turning them in or should I just sell them?

Do you have Blue Building access? You need to complete a treasure map as part of the entry requirements. I'd sell them otherwise.

What about Omen Island?

You don't need to complete an Uncharted Land map for Blue Building access but that's the only way to gain access to Omen Island.

But is it worth it to get to Omen Island? What's there? More quests?

The Ocean Depths vending machine where you can get newly released restockables and a Riptide, Merana's Quest where you can make your pets Hydrus, and Port Plunder where you can get pirate foods. I'd say it's worth it because the pieces are significantly cheaper now than they have been, and I'd personally be annoyed being blocked from a part of the Subeta map.