You'll appreciate that international typing standards were defined long, long before you tapped out your first swear word. Pitman courses trained typists to use 2 spaces after period and colon.I had never, ever heard of this 'two spaces' thing before joining AW and I learned to type on a typewriter.
The problem is when the writer is inconsistent. A space here and two spaces there. It makes the typesetter's job very difficult to conform to house style. The "search and replace" function doesn't always work so well.
It's a single space after a colon, period, question mark, or exclamation point.
It's only important in that it helps you look like a professional.
That said, if a publisher or editor asks you to use three spaces, and Comic Sans 19 point in flamingo pink, then by golly, you do it with a smile and be ready to grab the check.
It doesn't bother any typesetter I know. Spacing is the one area where nothing in the manuscript matters in the least to the typesetter. If you quadruple space between sentences, paragraphs, or anywhere else, his job is to simply follow house style in these areas.