There is no reason, in terms of story, for Stranger Things to take place in the '80s. But the '80s atmosphere - the dial phones, the chain smoking, Winona Ryder - are there to clue you in on the inspiration for this movie, which is pretty much every supernatural movie from the 80s with a focus on kids. E.T., Poltergeist, that sort of thing.
The wondrous thing is how dead-on this tribute. It's not just the clothes and hair, but the style of acting, the scripting approach, the structure, all hold true to their inspiration. You've got the young nerdy kids, the horny, unsure teenagers, the monster, the mysterious one with the power, the bad science/government people, the conflicted investigator, the tearful mom.
You could argue that this entire season is simply a pastiche made up of recycled elements, and I wouldn't disagree, but it really feels less like a copy and more like something of that era, as though the writers had fallen into a coma in 1985, woke up and got to working.
While it doesn't have the high style of the Spielberg films that are a primary influence, it does have the likability. The story is consistently engaging and fun, the acting is excellent, and the movie has sufficient laughs and tears to satisfy.