It lets us finally see the mysterious traveling big sister who introduced out protagonist to the classics clubs with her wish that he carry on her tradition. Then it shows some character development as he overcomes his melancholy disposition and comes to a higher social energy level through interaction with the Classics club and reflection.
Also, it stays away from the cliche of excessive fan-service. For better or for worse, it is hardly any more ecchi than the main series. Yes, they are swimsuits but there isn't the over-the-top in-your-face boobage and such like that you would expect from a typical swimsuit episode.
Rather, the characters are true to their nature in the series.
If you are a fan of the main series, then you really should watch this right after #11. It will make so much more sense that way!
