Actually, no, it didn't "officially" come between them. It was entirely made up by the Anime Team to kill time to let Slowpoke McMangaartist get far enough ahead in the manga they could continue the seires without running to a roadblock.
Nice planning so far guys.
It was added on the fly occupying an undisclosed timeframe between the end of the Soul Society Arc and the Hueco Mundo arc.>>11684650
Unless this is supposed to be a "Flash Foward" of events AFTER Heuco Mundo, then they are indeed taking a break from the Main action to tell a story which occured some time before the infiltration of Hueco Mundo.
As to why they had to do this, I'm guessing it's because of a differnece in pacing between the Anime and Manga, which is still ongoing. Because the manga is not progressing in a way that condusive to making multiple epsodes without having to "dragonball up" and spend ten minutes of constipated yelling per ep to stretch them out, the anime progressed a faster pace than the manga, resulting in the animators running out of fodder for a full season. Since they didn't want to commit to making, say, a quarter season of new eps with no idea where the story was going to go, they oped to create a new filler arc and kill time to let the manga get a headstart again.