This is analogous to adding uint8_t when we already had unsigned char. In C these would be exactly the same; in C++ they are different types. Same with uint8_t vs. byte: the former is an integer type, the latter is not. (Thus, a better question would be, why introduce byte when we already had unsigned char. I think, the answer to that is in a general tendency of moving away from the C way of looking at types and making code better reflect the intent and do it in a more type-safe manner.)
Overloading and templates. I can now use unsigned char, uint8_t and byte as distinct types, meaning they can be separately overloaded and used as separate template specialisations.
That's not a purely hypothetical point; I already create custom types to do this. Not every 8-bit type is a character, nor is it necessarily an integer. I always found it frustrating that the default stream output was a character when using numerical quantities; now we can specialise raw output accordingly.
Many programs require byte-oriented access to memory. Today, such programs must use either the char,
signed char, or unsigned char types for this purpose. However, these types perform a “triple duty”.
Not only are they used for byte addressing, but also as arithmetic types, and as character types. This
multiplicity of roles opens the door for programmer error – such as accidentally performing arithmetic on
memory that should be treated as a byte value – and confusion for both programmers and tools.
Having a distinct byte type improves type-safety, by distinguishing byte-oriented access to memory from
accessing memory as a character or integral value. It improves readability. Having the type would also
make the intent of code clearer to readers (as well as tooling for understanding and transforming
programs). It increases type-safety by removing ambiguities in expression of programmer’s intent,
thereby increasing the accuracy of analysis tools.
such as accidentally performing arithmetic on memory that should be treated as a byte value
My reaction to that can be summed up succinctly as "WTF!?" The whole point of uint8_t or (signed/unsigned) char is an 8-bit quantity that you can do arithmetic and bitwise operations on. To put it more bluntly, "have C++ programmers forgotten how computers work?"
The proposed solution is to add yet another same-yet-subtly-different type, with its own set of same-yet-subtly-different rules? If anything that would cause even more confusion due to the complexity it causes in interactions with all the other parts of the language.
IMHO this "let's do everything we can to stop people from even the very slightest change of possibly doing something wrong" line of thinking is ultimately unproductive... and actually rather dystopian. The end-result is quite scary to contemplate.
(The fact that an 11-page, text-only PDF somehow turns out to be over 800KB is somewhat less disturbing, but still notable.)