This ONA consists of 6 short chibi animations about the anime: "The Irregular at magic high school" and explains some of the concepts in the anime. The animation with chibi characters is plain bad and there are only two reasons to watch this as far as I can tell. The most important reason you would watch this ONA is to understand concepts from the anime since they're explained a bit better here. If you're just watching to understand a certain part of the anime then you don't have to watch any of the other episodes since there's no storyline. The episodes explain the followingconcepts:
1. Magic High school
2. Magic
3. Magicians
4. The Nine schools competition
5. Spirit magic/ancient magic
6. The thesis competition
There's also a 7th episode about the powers of Tatsuya which is listed as a sequel to this ONA.

The other reason to watch this ONA would be because you like the characters and their interactions and want to see more of them. The interactions are a bit watered down but every topic is presented by different characters and could be fun to watch if you really like them.

Overall this is an okay ONA because it bothers explaining concepts from the anime. However it is not something worth watching unless you want to know about some of the concepts or really like the characters from the anime.