Why the confrontation with the Bradys?

As we have learned countless times, things are never as rosy on the sets of these sitcoms as we would like to believe. This one was really more the story of Danny than the total sitcom.
I don't know if this was just a bunch of bad actors and acting or I was subconsciously missing the "real" Partridge family. Maybe it was both. Still, the insights were interesting although Shirley Jones just seemed too good to be true.

However, I don't understand why the director and the producer felt the need to have that "showdown" between the two TV families, the Partridges and the Bradys, and why they kept it secret from the actors untill they met. Were they planning to have guest appearances on both shows? Nothing was ever explained or discussed nor was any aftermath mentioned. It all looked very awkward and maybe that was the point. Does anyone have a guess?