It is also about distance from the person you shoot. With that length you are far enough away to have room for lights, flashes and such, but not too far so you have to shout for the portraitee to hear your instructions (tilt head to left .... nooo,. the other left ...). Any shorter focal length will put you almost "in their face" which is not good from several perspectives: bad breath, no room for lights, depth of field can become an issue.