Congested status was made back when people were flooding Balmung. You couldn't make characters there most of the time except the time of lowest activity in the week during the night, so the most common way to get in was to transfer there. And there were hundreds of people transferring there each day. I know because I was recruiting for something before cross-world PF and all the people new to the server sent me a tell.

It was unsustainable that a congested world was still being flooded so they created the Congested status that has the additional effect of blocking transfers and have since used it on many other worlds now that the game has a much larger population.