That’s a function of typewriters, when each of the characters took up the same amount of space on a page. The double-space made it easier to find the beginning of sentences. It’s how I learned in school (we had typing in seventh and eighth grades), but modern proper form calls for single spaces after sentences.
I still double space after a period. It's the way I was taught and I've been doing it that way for over 40 years. No one has ever even mentioned it, let alone complained about it.