This usually takes some practice for novice singers in the studio, They may not be used to hearing electronic room tone, and may think being too loud in the title is intimidating and so sing with weak confidence, when they would be happier faster to just belt it out. And they may ask for too weak a headphone level or not enough band to get their pitch reference. So getting a great headphone mix is quite important new singer or old pro. These are the main reason for using headphones. Nowadays there is a large usage of in ear monitors for live work, where the singer can get their ‘personal ideal room tone and level’, and as much of the band as they wish and sing better for it, with an over all quieter work environment and less risk of hearing loss.