It used to be taught in each and every typing class. It has fallen into
desuetude in recent years because it normally looks clumsy in
proportionally spaced fonts, and, in some software, leads to oddities
something like the following example (which I've formed artificially).
 See how this line looks? That's the problem. In fact, this would be
all over websites, if HTML did not have a specific rule that /all/
whitespace sequences, of whatever size, are automatically reduced to a
single space.

There is something to be said for sticking to the old rule when, for
whatever reason, one is working in a monospace font, but it's very
difficult for most touch typists to teach the fingers to follow two
different sets of rules.