So why wouldn't they just have the denomination embossed in braille?

This implies actually knowing Braille. Even in very developed countries, not all blind or visually deficient people know Braille. Arguably, they should be pushed toward Braille rather than left in a "comfort" zone.

There are many ways for Braille to fail, particularly if embossed on a regularly-used banknote, and not everybody can use it. Differently-sized notes are much harder to break, and are potentially useful for everyone. Make the notes differently-coloured as well, and they're straightforwardly distinguished by most people, regardless of disability (or lack thereof), in a variety of situations.

(I would imagine Braille costs a fair bit to print, but banknotes are somewhat expensive to make - compared to most pieces of printed paper, I mean - so perhaps that wouldn't be an issue.)

Because when you are eighty years old and you become progressively blind, it's difficult to learn a completely new 'tactile' language.