For the most part when we were typing on paper we used fixed width character fonts. Two spaces made it easier to see the end of lines.
Now a days we use proportionally spaced fonts which is easier to read. Access to typesetting software took a while to spread, which is why the habit was slow to die off.
I actually used a linotype machine for a while to help learn how to typeset. I worked on one of the first newspaper publishing systems for the Chicago Times. The system wrote directly to a descendant of the linotype and could write an entire slug for a story column in one pass.
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(Don't blame me, he started me rambling!)