I've been thinking about the question raised in the OP (what was the point of a North facing bay at Bletchley?) and accessing the recesses of my memorybank back to trainspotting days up to 60 years ago, and I think I have the answer.
I'm pretty sure that the track concerned was originally a through track, and the platform extended to the same length as the other platforms. It was used for Bletchley - Oxford services.
When it existed in this form, the station building was (obviously!) East of these platforms but when the new station block was built it was constructed across the middle of this formation. Presumably, the reason for this was so that the existing station facilities could remain in use until the new building was complete. I'm not sure when this happened, though.
As an aside, the line now referred to as 'Oxford - Cambridge' it was largely operated as two services, one each side of Bletchley, possibly to avoid crossing the main line.