What are the uses of a high quality 50mm lens?

"High quality" is a always a dangerous term because it is a euphemism for expensive and various other things. Early in my working career I was  a company which had embraced Japanese-style quality philosophy and we got taught that "Quality is meeting the customers needs".  The product which the customer can't afford doesn't meet their needs.

Some customers need a lens which is robustly manufactured and endure the pounding the professionals give their equipment and not fail. Some need to extend that to working in extremes of temperature, moisture, dust etc.

Some prefer small and beautifully engineered (like the 3 amigos) to large lenses even if it means accepting compromises in image characteristics. [I'm in this camp]

Wide aperture is not "quality" of itself but if you work in low light conditions and or you shoot a lot of portraits where you want to separate the sitter from the background, an extra 3 stops makes the big difference in whether it meets your needs or not.

The same is true of AF, speed and noise may be irrelevant or may make the difference between meeting your needs and not meeting them.

There a plenty of people who like the way old lenses render, though the colour accuracy and sharpness leave something to desired. Others want the ultimate in detail. I bought and returned the 15-30 because I couldn't live with its mass; the arguments on here of late show that I can't think in the mindset of those who would carry a lot more bulk to get the last tiny bit of detail / sharpness (what is the delta between my mid-range tripod and one which weights another 25lb / 10KG ?) and they can't see why someone would compromise on image quality to save a bit of weight. If we're going to get along we'll need to accept the validity of p.o.v we don't share or even understand. Initial sales reports of D-FA * 50 suggest there are a lot more people who prioritize that extra quality than I first thought.

Then you have the question of what focal length; and what I have come to think of as the zoom fallacy. A prime is lighter than a zoom (doh!) and only an expensive , heavy zoom gives a wide aperture. (I've never seen a zoom wider than f/2, you can get an f/2.8 which is equivalent to 28mm f/2.8 prime or a 135mm f/2.8 prime). A prime might well offer better image characteristics.  But a zoom saves having to change lenses in adverse conditions, and might save carrying half a dozen primes so gives a lighter camera bag.

But here's a question  a 28-105 is a 28 , a 29, a 30 ... a 104 and a 105 all in one, you can always you the optimal focal length. But your 28-105 stats (and mine) show more than half the pictures are taken at the two extreme focal lengths (you'd use longer or wider if you had them) So a 28, a 135 and a 43 or 50 or 77 might be a better solution