ASCII (as opposed to ISO 8859) only uses 7 bits per character, and one byte per character. Byte numbers with the highest be set to 1 would have value 128-255, which is undefined by ASCII.
Some (old) serial communication protocols use only 7 bits per character. In such a protocol you wouldn't even be able to send the 8th bit because it isn't there.
It is always good to remember that in general a char is not a byte. Only in ISO 8859 this is the case. But in UTF-8 for instance, a char is 1, 2, 3 or 4 bytes.