The typing habit was designed to assist typesetters in reading the manuscript as they were setting type. Traditionally, the typesetters never set the extra space.
Apparently not universally true (see my other mail).
Also, when lines had to be stretched to fit, manul typesetters would not treat all spaces uniformally.
Finally, the majority of typescript documents were never typeset. Nevertheless, the rule about the extra spaces after periods is apparently not limited to documents for typesetting.
A./
