While I'm sure there was an idiot or three who actually tried it in the old west, Fanning the sixgun is a movie trick. Even folks who recognize that its only done in the movies (and TV) don't usually ask WHY its done in the movies.
we all know why we don't do it in the real world (other than planned exhibitions), because its hard on the gun, can be hard on the shooter (fan a Ruger Blackhawk
), and unless you are a natural talent (rare) or have shot a wagon load of ammo practicing from the hip, you won't be hitting your target.
It is done in the movies, for the dramatic eye catching movement. And that's all. They are shooting blanks, they are shooting prop guns (not their personal property) so who cares if the gun gets battered, or even breaks, as long as it lasts for the scene being shot.
The WORST gun handling (in terms of what's bad for the gun) is in the movies and TV, though to be fair, TV just upheld the "traditions" established in the movies.
Fanning the SA revolver, "Bogarting" the DA revolver (flipping the cylinder open and shut), flipping the double barrel shotgun so it slams shut, and others. Visually dramatic. Director's love it. Bad for the guns, but good for the picture, and the guns are just props, so who cares??
Chuck Conners spin cocking his Winchester as he walked was very dramatic and eye catching, very impressive.
Of course, the rifle had to be specially modified so the blank ammo used wouldn't fall out of the gun as he spun it, And you had to be as tall as Conners to spin the rifle, without hitting the dirt with the muzzle....
Think of fanning as "movie special effects" and not something to be tried at home, or the range and you'll be better off, and so will your guns.