I think this needs better highlighting.

Because Kanban is a pull system, you can't simply move the card to the next work phase when you're done with it. It's important that whoever is doing the next work phase be able to pull the work into their queue when they are ready for it. That means they need to be able to distinguish between work in your queue that is still in progress and work that is ready for them to pull. For that reason, each of the columns are split into "in progress" and "ready".