Bakers, let me ask about flour

I’ve been eyeing the King Arthur recipe for salt bread:

https://www.kingarthurbaking.com/recipes/salt-bread-shio-pan-recipe?_gl=1

This recipe calls for both bread flour and cake flour. My understanding is that bread flour is high in protein and cake flour is low in protein, so by mixing them, you’d get something more medium in protein, which is what all-purpose flour is. So what is the logic behind using these two flours rather than just using all-purpose? I’m trying to decide if it’s worth my while to buy cake flour, which I’ll likely never use again.