What if somebody told you that there is a note you didn't know about, an entire tonality you never heard or used? That is what C# tonality was like during Bach's time - seldom heard until the Well Tempered Tuning.
And to introduce it well to the listener, Bach makes the scale ever more prominent. He first passes through the C# M scale, then the relative minor, then shows off an exrecise in modulating to the Major scale on the Dominant, before he makes a full return to the tonic scale. Honestly, all this erudite in tonality makes it sound so Mozartean and Classical..