The main joke, as someone else pointed out once, is that Brian, normally the calm and rational voice of reason, is introducing such an absurd, ridiculous and out of nowhere character. And in such a calm deadpan manner at that.
I guess I could conceivably be wrong about this, but I highly doubt that that is the only time that Brian has been the lead-in to one of the cutaways.
He actually quite rarely does, and when he does it's almost never something as absurd and out of nowhere as that was, or in a deadpan way.
In other words, it's an indication of sloppy writing/characterization. No need to overanalyze.
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