If you haven't watched Hakuouki & Hakuouki Hekketsuroku than it's important to watch them first in order to understand the more intricate details about the overall story.
I love this because it shows us the boys a bit more and how their relationship with Chizuru is, and why she's important to them (the game that the anime is based on is an otome one, afterall!) and it also gives us more light-hearteded enjoyment as well, since the main series is a bit heavy.
Plus, we get to see how these characters are not flat copies of one another, or just trope-y otome love interests, they are interesting, well rounded individuals that all have their own flaws and strengths. I think it's a very good addition to the collection of already great works for this series.
