It's a thicc bit of dough and the hole is for surface area when baking. Otherwise you'd either get a raw center or burned edges. Also they're not just bread rings, bagels are a different process which involves boiling the dough prior to baking and other stuff depending on the region (Montreal bagels>NY bagels btw). They're not really intended as sandwich material and that's a more recent use for bagels, that being said I fuckinf love a bagel sandwich.