We have an ear on each side of our head for a reason, so we can hear sounds from 360 degrees all around us. With two eyes we can only see what's in front of us -- we have to turn to look in other directions -- but without turning, our two ears can easily locate birds high up in the trees, a car coming from behind or someone calling our name from the left side of a room. I'm guessing the prehistoric humans who didn't hear the bear or snake approaching from behind didn't live very long. It makes sense to listen in stereo.
Do you hear stereo?
From the earliest Thomas Edison recordings in the late 1870s to the late 1950s, music recordings were all mono. It was followed by stereo LPs and tapes whose popularity soared in the 1960s. This means stereo has been around a really long time, but I've noted most non-audiophiles rarely listen to stereo over speakers. To do so requires sitting in the "sweet spot" where you're equidistant from the left and right speakers. If you're closer to one speaker than the other you'll never hear what the engineers were going for in the mix.
The amazing thing about listening from the stereo speakers' sweet spot is, you don't just localize the sound from the two speakers, you hear "phantom" sounds between and behind the two speakers. Vocals on most recordings will appear centered between the speakers, and that effect can be startling the first time you hear it. You might start looking for a hidden speaker in the middle, that's how convincing phantom sound can be.
Some sound might even come from farther to the left of the left speaker and to the right of the right speaker. Depending on the mix you might hear sound coming from behind the plane of the two speakers, which produces the illusion of depth. Or maybe you'll hear sound projected forward of the speakers' locations. Stereo, when it's done right, sounds three-dimensional.
Of course, with headphones it's hard to miss the left-to-right placement of instruments and vocals. They might mostly appear inside your head, but over the best headphones you'll hear depth and some sounds might appear to come from outside the headphones. When you listen over speakers you hear a combination of sound direct from the speakers mixed with their sound reflected off the walls, floor, ceiling, furniture, windows and mirrors in the room.
As always, the more you focus on listening, the more you'll hear. For example, Radiohead's recordings sometimes have complex spatial mixes, but you might not have noticed them over speakers if you weren't in the sweet spot, and really focused on the sound.
So stereo "imaging" isn't merely the placement of vocalists and instruments in a straight, flat line in a row between the left and right speakers. You might hear that the guitar for example is closer, more forward than the drums, which sound farther away, more recessed in the stereo image. The sound of a vocalist might also be forward, but surrounded by reverberation that's more ambient within the image. Each instrument might appear to be fully formed, even three-dimensional within the stereo image.
Summing up, to properly hear the stereo mix you have to be equally distant from the left and right speakers. Of course, the speakers' placement within the room affects stereo imaging. For example, if your speakers are too close to the wall behind them the stereo image will be flat, and moving the speakers out into the room will deepen the image.
Of course, you might not care enough to bother with any of this and that's fine, but if you really love music, try some focused listening time and let us know what you think in the Comments section below.