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.)