The exact length of a word varies. What I don't understand is what's the point of having a byte? Why not say 8 bits?
Even though the length of a word varies, on all modern machines and even all older architectures that I'm familiar with, the word size is still a multiple of the byte size. So there is no particular downside to using "byte" over "8 bits" in relation to the variable word size.
Beyond that, here are some reasons to use byte (or octet1) over "8 bits":
1 Although I'll consider a byte to be 8 bits for this answer, this isn't universally true: on older machines a byte may have a different size (such as 6 bits. Octet always means 8 bits, regardless of the machine (so this term is often used in defining network protocols). In modern usage, byte is overwhelmingly used as synonymous with 8 bits.