Baking in a rectangular shape allows you to make a space efficient bread that you can easily stack and transport. Also, it is very predictable, can fit neatly into your toaster, and can be cut in triangles.

Making bread on a flat surface allows you to minimize costs of entry (not only don’t you need the forms which are relatively cheap, you can go with simpler/cheaper ovens), and this kind of bread has a more pronounced crust, which many people like.

Also, rectangular bread is harder to leaven for a long period of time as it comes with numerous technological complications down the production line. This affects the aroma composition, making rectangular bread less attractive for those who want the traditional “bread” taste.

Baguette, as I already mentioned, has a unique crust and crumb texture defined by the shape and baking conditions. Many people like it that way.

The containers can increase your breads toaster compatibility :D

But overall I guess it’s a bit like pasta: different use cases (sandwich, sides for salad or soup, as a stand alone dish / food), regions with different resources for flour, fluids, spices, … and also different kinds of utilities (metal pans weren’t easily available everywhere, all the time and they take up space) and so on.

And all these things influence how the bread tastes, looks and feels. So variety in process (container vs surface, loafs vs flat breads, handcrafted vs automatically processed, …) leads to different results with different characteristics.

E.g. I love Apulian bread. It’s a loaf with a slightly darker crust, but lighter and soft on the inside. The crust gives it a slightly bitter taste, that makes it a bit rustic (the only thing better is a fresh baked sourdough loaf).
It’s perfect for sandwiches IMHO but for french toast it’s a pain in the ass. I use pan baked toast (different density, crust and form) instead and again: perfect bread for this dish.

And then just imagine eating a Döner from half a loaf of grey bread, or toast, … blasphemy!