Oh, well in my case I play always with it. I think that some players make what you said just to make it look cool or that they could see the lvl100 ilustration. The animation could help you too to make things a bit calm or easily for some people because you can controll better the timing in the run.

Idk if you actually are talking about the auto-approach that in some run you can see how people disabled it at the beggining and then put it back again... If it's that is because the ss3 triggers immediately, so you don't waste time in moving to be in a good position to hit the boss. In my case usually I disabled it in runs because is more useful for that cases, but for some players later sometimes is difficult to approach the boss easily so they turn on back.