I am under 40, but had "two spaces after a period," drilled into me as a kid. I only broke the habit in the last year, and it still feels weird every time I use just one.
Usually you aren't given a reason for not doing something, unless it's dangerous to do the thing. 🙃 But if you're instructed to do more work, it can be reasonable to assume someone once thought there might be a reason to do the extra things.