Speakers need room to breath, headphones do not. If you wish your loudspeakers to image, it's unlikely they will do so shoved up against the wall behind them. There have been a few designs that allow imaging when up against the wall, I believe Naim Audio designed one. But, for the most part, speakers must be pulled away from the rear wall to image properly.
Headphones are a different story than loudspeakers, and take some getting used to. Most of us are so accustomed to the speaker's presentation in our rooms that when the headphone's earpieces envelope us and close off the outside world, it's often helpful to close our eyes so our brains can readjust to the idea of illusory depth measured in feet, from sources only inches from our eardrums.
Of course it's all an illusion. There is no real depth, no physical space required to generate one. The illusion of a soundstage, of depth, is enhanced when we turn the room lights off, or close our eyes with both playback mediums.
Illusions generated by speakers are fundamentally different than those presented by headphones, yet both start with the same information etched onto the recording. Speakers involve the room and the space they play in to build an image. Headphones isolate us from what speakers depend on. Their presentation is a true function of what's on the recording, sans external contributions.
I still prefer loudspeakers over headphones when it comes to convincing me music's being played live, but both have their advantages: headphones for close examination of minute details, loudspeakers for the bigger picture with visceral impact.
