This mod was initally part of CypRyan's Wide and Tall mod, which enables playstyles where you focus all your resources in a few cities as opposed to many. The original Wide and Tall mod gives you extra growth modifiers the fewer cities you have, so if you played tall you would have the population to work the extra tiles given. The workable outer rings also comes in handy when your cities have a lot of districts/wonders, which reduces the number of tiles you can work. Another use case is for when you are planning national parks, since a lot of the time you want them near the edges of your cities and this mod opens up more room for them to be placed.

Thank you for the explanation!

Though I find most districts actually increase the population needed to work every tile. Every building can hold a citizen so that can be 3 population in 1 tile.

I'm still not sure how this would work well with the loyalty mechanic, but I may try this mod sometime for border cities. Sounds fun, especially for National Parks!

Couldn't have explained this better! :)