Because for someone who rarely do bdsm/bondage scene (mine included) might get uncomfortable if some parts is touched by someone that isn't part of the pre-play agreement, have a phobia of something or for whatever reason. So comfortness is also playing a key. And sharing is also important.

Make sure both the subs and the doms agree on what scene you/they want to play beforehand (the basic consensus before play). Sometimes subs or doms are reluctant to do some type of bdsm/bondage if the resks are too much. And I prefer asking to the subs or dom if they are comfortable to do it.