If you want to play any hitscan based FPS at a high level, you need to be able to do this consistently. The drills I intentionally go out of my to keep the croasshair a bit off, so in a real game it looks a bit different. The idea has two parts to it. One is that no matter where your crosshair is on screen, if somebody enters your vision you can very quickly throw your crosshair on them and land a shot. The other is that in a game like Overwatch, people can change the direction they are moving in instantly, so if you're trying to track somebody with erratic movement you can and will make mistakes. If you're just throwing your crosshair back on them instantly with every shot, you got a good chance to land hits.
The reason some players "snap back" with their cursor just has to do with crosshair positioning and muscke memory stuff on the distances you can flick.
Every time I post something showing off flicks I get the same response of "why would you gimp yourself playing like this" when there's a very good reason to be flicking often. I don't take literally every single McCree shot with wide flicks like this, but I take them often enough with smaller ones and some wide ones.
Like, if Pharah hits shift and I'm playing as McCree, I can instantly flick to her face where the shift will put her since it's predictable. Same thing with Genji shift. If somebody wide peeks me I also will flick onto them. Etc.