GrahamS had the correct answer back on the first page.
Double spacing is a throwback to monospaced fonts
where a letter m
and a letter i
are the same width. With modern proportional fonts it’s unnecessary and considered bad form.
As someone else noted, STW will strip out double-spaces; that’s actually a Web thing rather than specific to here, web browsers will strip multiple spaces when rendering HTML (unless you explicitly tell it not to in the page’s markup).
I can’t help but double-space, but then I spent over a decade using computers with monospaced fonts (from the ZX Spectrum to green-screen PR1ME minicomputers) and twice as long on Mono so it’s a hard habit to break.
When I text with my phone, keying a double space always inserts a full stop.
That’s just a convenient shortcut, you won’t actually get two spaces (unless you press it a third time).