Why should you play Chrono Belt, fans of Ayakashibito and Bullet Butlers?
This is your reason. Do you really need another?
But seriously, this fight is told entirely from Kuki's point of view, and as a result it is suitably awesome. Rather than doing it in order, I went ahead and did the Kuki-viewpoint segment first solely so I could re-experience the awesomeness that is Kuki Youkou.
Anyway, Chrono Belt isn't so much a fandisc as a true-ending crossover of both games. Basically, it happens after the ending of both games, and it is a VN that manages to standout on its own as awesome, though it is reliant on the characters from the two games for its plot.
To be blunt, this VN manages to satisfy fanboy crossover 'what-ifs' while not being ridiculous, which is a feat in and of itself. I'll leave it to you to imagine what those what-ifs are, because, as always, I refuse to spoil it for you. I will say that there are three segments: Kuki, Alfred, and - last of all - the main segment. All three are generally cool and interesting, though the last one is kind of heart-breaking or stomach-twisting at times, for fans of both games. I won't be doing a complete rundown of this VN, because it is quite simply impossible to do so without spoiling its origin-VNs.
PS: Incidentally, this is one of only a few fandisc-type VNs that I've played/read that weren't excuses for extra H-scenes (though there are extra H-scenes, lol).