The OVA is a good "snack" to have before the release of the new season. If you have binge-watched everything up to this point it might be a bit boring I guess. Since it has been some time since I watched anything of "Haikyuu" it was quite nice to get a refreshment with some flashbacks here and there.
It adapts basically the chapters 190-199 which would be vol. 21-22 (of currently 44).