The purpose of hreflang is to communicate to Google how they can access different language/region variations of a web page. If your page is only in one language, then there is no need to use hreflang. Common sense, no?
A webmaster asked John Mueller this question and John answered on Twitter that "it doesn't make any sense to use hreflang if you only have one language / region that you're targeting." He said, it is "easy," don't bother adding the hreflang to your site because it is unnecessary.
it doesn't make any sense to use hreflang if you only have one language / region that you're targeting. Easy :)
— John ☆.o(≧▽≦)o.☆ (@JohnMu) June 7, 2017
Like this site, I only write in English (if you call it that). If I translated the pages to Hebrew, Spanish, Japanese, sure I should then use hreflang. But I don't, I write it in one language, so I don't use hreflang.
Forum discussion at Twitter.
Update: The title of this story was updated to reflect what was written in the body. I attempted to go shorter with the title and it caused some confusion, I am sorry.
