In nearly every venue there's a sweet spot: somewhere that maximizes the intended experience.
In a live concert, it may be the center, row three. Or perhaps the right side of the orchestra if you like the fuller, richer sound of the cellos and bass.
In a movie theater it could be dead center middle, or up a little closer.
Most of these choices are personal depending on our tastes.
But in a two channel high-end system, the sweet spot's right dead center where you set it up for best imaging and rarely varies from person to person. This is because in a two channel setup, the three dimensional illusion (phantom center channel) we're trying to generate is so heavily dependent on both ears getting an equal dose of audio from both speakers.
Because the size of the sweet spot in a two-channel system is completely dependent on the left and right ears maintaining equal volume from both speakers, how do we control the size of the area? By sacrificing something else like depth or image beyond the loudspeaker boundaries.
Remember, the whole premise of two-channel audio is hopelessly flawed because we're trying to generate the sound of a single point source from two speakers. It's a magic trick and move too far out of the boundaries and you see the magician's secrets. But two-channel is all we have right now.
So, let's imagine the extremes we have to work with. One extreme would be to place the two speakers as close together as possible. This would reduce the image between the two speakers to almost nothing.
The other extreme would be to place them as far apart as possible, then point them in towards your seating position. This will create the largest possible sweet spot. Then there's everything in between.
The smartest thing to do is define the size of the sweet spot you want. Let's suggest a three seater is best. Then move your speaker pair apart wide enough to encompass your intended sweet spot and tweak until you get the whole system dialed in - realizing all along you'll give up perfection for sweet area.