I, as a user, do not need any files in /usr/share/apps. The system might need. And I don't want to deal with those.
All other major operating systems (can) work like this; download something > click it > it works. No need to launch terminal, set executable permissions and type the name of file.
Make it executable
$ chmod a+x Subsurface.AppImage
and run!
$ ./Subsurface.AppImage
That was easy, wasn't it?"
No. How about click/double click the app icon in your menu/desktop/the folder where you downloaded it into?