You occasionally still see two spaces after a period, because people tend to do what they were taught in school.

But it actually has been obsolete for more than 100 years.

People began using two spaces in the late 1800s because the printing press (1450) and even the typewriter, invented in 1870, were sometimes inconsistent in their clarity. The impression on the page was not always distinct, so readers might think that two sentences ran together. To counter that, people began using two spaces to create a greater separation between the sentences to ensure easy readability. But as David Jury says in his book, “What is Typography?”, improvements in the typewriter soon rendered the practice of two spaces unnecessary.

Some people feel strongly about the issue, however, so they continue to use two. It is not wrong, but it is considered nonstandard now.