Using the ob itself (its width/diameter) makes it pretty simple to visualize fractional partitions as shown here. Anyone with the slightest spacial skills can accurately estimate a vertical ob center line and far left or right vertical line, which makes it very easy to pinpoint the halfway line or point between these two visualizations. The example below is a 3/4 aim.
Naturally, unless there is a specific marking on the ball at the exact aim point or vertical line, this isn't something you can literally see. But compared to an imaginary ghostball, at least there is something real and plainly visible (ob) to use as an aiming reference for visualizing the aim line, aim point, or vertical slice of the ob.
Ghostballs are invisible/imaginary. You must visualize where the ghostball needs to be. You do this using your best estimation based on experience, then you must visualize an aim line that puts the cb at this estimated ghostball location, referencing absolutely nothing other than your imagination. If you are off on visualizing either one (the ghostball itself or the aim line), or you can't deliver the cue properly down this line, then you're likely missing the shot and will not know if you missed due to a bad estimate of where the ghostball should've been or a bad estimate of ligning up to this invisible reference point. Or maybe the miss was due to a flaw in stroke mechanics.
I believe the more defined we can make our aim (like using the ob as a reference or guide, either in conjunction with an estimated ghostball location or simply ignoring the ghostball and focusing directly on a point or slice of the ob), the quicker we can program our brain to recognize shots as soon as we them.