It Starts with Geometry
From the centre column, you view the screen straight-on. Move to the side and the rectangle keystones into a trapezoid - one edge stretches, the other compresses. Curved screens (IMAX, ScreenX) exaggerate this. A centred seat is the only place the image keeps its intended shape.
Sound Is Mixed for the Center
Cinema sound - and especially object-based systems like Dolby Atmos - is calibrated for the middle of the room. Sit there and left/right channels are balanced and effects pan correctly. Sit on the far side and you're much closer to one speaker array, collapsing the soundstage. This matters most in Dolby Cinema.
Why it's called the 'director's seat'
Center Matters Even More in 3D
For 3D, horizontal centring is critical: off-axis seats skew the left/right stereo images so depth flattens or doubles at the edges. It's the single biggest factor for clean 3D - see best seats for Avatar in IMAX 3D.
Are There Exceptions?
Rarely. If the only centre seats are in the front row, a centred seat one row back from ideal still beats a perfectly-distanced side seat. Distance is adjustable; a skewed angle is not. Confirm your pick in the 3D simulator.