What happens when I select 'disable and download messages' for iMessage on iPhone?

I want to keep all my messages in the Cloud, but I don't want any stored on my phone. When I go to Settings --> iCloud --> Saved to iCloud --> Messages in iCloud and deselect "Use on this iPhone" a message pops up that says "cancel" or "disable and download messages". What happens if I select disable and download messages? I'm assuming it will turn off imessages syncing to the cloud and download all my messages to the phone? Why is that the only option?

I don't care about future messages from this phone syncing to the cloud, but I do want to keep my message history on the cloud. I just want to use iMessage without any prior chat history on my iPhone, if there's another way of doing this that I'm not considering.

[Re-Titled by Moderator]