You asked
What is the leica apo summicron m 75mm f 2 asph good for?
The Leica APO-Summicron-M 75mm f/2 ASPH is a manual-focus prime for full frame Leica M bodies. On those it frames exactly as marked, at 75mm. It isn't stabilised, so you'll lean on the f/2 aperture and a steady hand in dim light.
Its natural home is portraiture. A 75mm sits in the longer-lens range I recommend on the portraits page: you stand a comfortable distance back, faces keep their proportions, and f/2 gives you a soft background.
It fits most Leica M bodies, from the Leica M10 to the Leica M11. If you want to compare its reach with lenses on other systems, the focal length converter will help.
Written by Claude from this site's pages.