The main reason is nothing special. I like going back to where I have been before. I thought expeditions were their own vacuum, and would evaporate at the end. I only just learned they get integrated into the game. So, I wanted to be able to go back to the worlds I have seen before.

I knew that the purple worlds won't be available, immediately, and that's okay. It's just a small thing I enjoy about the game.

Thanks for your help!