Cuz sometimes patching only one input or output has some kind of function. Like, for instance, on the Mother-32, patching only EXT.AUDIO Input without connecting the end of the cable to any output will mute the White Noise on the MIX control which unpatched will gradually increase to maximum Noise when you turn the knob fully clockwise.
In your specific case these "dangling" patch cables obviously has some kind of function. But to be honest, I don't know myself what function patching these specific outputs without connecting the cables to an Input patch point has. But just because you made me aware of this I will now go and try to find it out : )
Maybe someone else knows?