So, I'm guessing what this episode is is a section of the manga that was more fleshed out than what was shown in the series. In other words, this is an episode that they couldn't fit in the main lineup.
If you've seen the second season of Haikyuu!!, then you'll have seen certain parts of this episode already. All this episode does is fill in the blanks. I don't think it's an absolutely essential OVA to watch, but I do think that it provides a little something more to the characters.
Tanaka is the first example that comes to mind because you'll see him in
a more serious light with this episode than what you'd normally see. You'll also hear him gives some honest opinions on the first years, which is rare. Tsukishima is the second example I have because you'll learn how he deals with his fellow first years and how much the freak duo really means to him as a member of the Karasuno volleyball team.
In other words, there is actually character discoveries in this episode, which is pretty cool for an OVA. Beyond that, you're just gonna get a funny episode of Haikyuu!! where you get to chill out with the characters without them being on the volleyball court. It's a nice extension to the bit of this episode that we see in the main anime, so if you really like the anime, definitely watch this.