Bottom Bs

[Vox Humana]

I've just been sent a copy of Darke's Magnificat and Nunc Dimittis in F ("Darke in F" to some, "F in Darke" to others). At one point the organ part has a B below the bass stave. Now one gets quite used to seeing such notes in piano reductions of orchestral accompaniments, but this is an original organ part (as far as I can tell). I don't think it's entirely unique either; I'm sure I've seen something similar in a pukka organ part at least once before. What I'm wondering is whether there was particular reason why composers wrote such notes (and if so, how did they perform them). Given that Darke belonged to the "orchestral" generation of organists, was he just writing an orchestral accompaniment that he expected to be rearranged in the same way one has to rearrange, say, Parry's I was glad? Or was he catering for choir rehearsals with a piano? Or what?