The original Purge was set inside a single home with people trying to survive the night. But then they spread out to the whole city, which was new. But then they just kinda change it up a little and rehash the same story. And this is just another rehashing of the same story. So, I guess if you liked the other movies, then you'll probably enjoy knowing more about the origins of The Purge. But it really is the same movie, so you don't REALLY need to watch it. You can imagine most of what is going to happen.
The First Purge tells the story of... yeah, the name says it all. Really. It's the whole movie. Some people hate the idea, some love it, eventually, chaos makes everyone crazy. They just try and survive. Some characters have revelations, the movie ends with a sort of triumph for some of the characters. Yeah, different characters and some cool concepts, but it's the same story as the other two 'in the city' Purge movies.
Had this been the second Purge movie, I would have enjoyed it more, I think. But since it's pretty much the same as the other ones, it just gets a little old. There are so many ways that Purge stories can be told, I think it's rather lazy to keep telling the same one. But if you're a completionist, you might feel the need to see it. But there really isn't anything new, so you can skip it.