NavList:

I don't know, but have always assumed that the 12x scopes were intended for distance-off horizontal or vertical sights where the observed angle can be very narrow.  So when piloting close to shore, and from a stable platform (such as a ship's deck) the additional magnification could help.

The same could apply to other marine surveying tasks and even to meauring distance from a ship at sea - as in station keeping during WW2 convoy operations.

I'm making this up, but with a distance between ships of 3 miles, the adjacent ships in the convoy would all be short of the horizon when sighted from a ship's deck.  Thus if height of the adjacent ships is known - say to a prominent point of the superstructure, funnel or a mast, then for a specificed distance that vertical angle is also known.  This would be a rather narrow angle, so a 12x scope would be a useful tool for determing whether a ship was more or less keeping station. 

I've never used one and indeed have never seen one other than in photographs.  They seemed to disappear from the "new" market by the time GPS was available as far as I know.  I can't imaging how I would use one for a celestial sight unless taking that sight from the land with the sextant fixed to a tripod of some type.

That said, at this link...

https://www.usni.org/sites/default/files/styles/hero_image_2400/public/Arms-NH-MJ-20%201-opener.jpg?itok=-p3eb7Af

... this picture appears.  The caption claims that the sight being taken is a sun sight.  The position of the index arm and the presence of the timekeeper supports this.  

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