Mainly because square slices look cool and are more practical.
1 . Square slices
The pan’s tall, straight sides and lid prevent your sandwich bread from “mushrooming” into a loaf whose slices can be hard to cram into the toaster. Instead your bread will be perfectly square: a fine-grained loaf ideal for slicing without crumbling.
Packing sandwiches for lunch or a hike? Square is obviously your most space-efficient shape. It also means the slices from the end of the loaf are the same size as those in the middle, a consideration if you’re making a bunch of sandwiches and want the slices to match up.
I like square bread. Looks aren’t everything, but I prefer my grilled cheese cut into perfect triangles, thank you very much.
I saw Julia Child use one of those pans with a lid but as an alternative she used an ordinary pan and covered it with a platter weighted with some heavy tiles.
The PBS channel @create has some of her old shows. She’s a riot.
BF has more protein than AP which makes it stronger and able to absorb more water so you can go a lot higher hydration with a high quality BF like King Arthur vs AP before the dough won’t hold it’s shape or starts to tear apart