When you close your eyes you can transform your surroundings, and become the person that you want to be. When you stand and sing up on stage, feeling naked before an audience hungry for music, I imagine you can blot them out by shutting your eyes and allowing the music and lyrics to take you on a journey, losing yourself.
You can stand before an audience of thousands, and be in an empty room, singing only for yourself. You can transfigure a handful of loved ones and seat them in the front row, directly below you, where you can see them smiling; hear them applauding. You can stand them behind you and feel their admiration burning into your back. You can place your love interest in your line of sight, and draw an expression of love and awe on their face. I'd imagine.
But for someone who merely sings in the privacy of their own room, or perhaps the shower, it is very much the same. You can close your eyes and the walls melt around you. You can stand on the rug in the centre of an empty room, and be on a real stage, singing your heart out for the countless people who have gathered to see you. When you sing in the shower you can simultaneously perform at Glastonbury, your audience watching spellbound, ignorant of the rain falling around them.
When I sing with my eyes closed (alone in my room, or perhaps the kitchen if the house is empty) I sometimes find myself with an audience, cheering and clapping for an encore. But I am always a talented and confident performer, with the voice of an angel. And sometimes I might miss a note only to have my fantasy crumble around me, leaving me feeling flustered and foolish.
I wasn't born a singer, and have no real desire to perform in front of an audience, but I can still dream.