A matt or flat finish scatters light diffusely rather than reflecting it directly, which minimises glare and can make a colour look richer and more even, but shows less of a sheen and can highlight surface imperfections in raking light.
Light striking a matt surface meets a microscopically rough film and is scattered in every direction rather than reflected as a coherent beam. Almost the same total amount of light comes back, but it arrives evenly from the whole surface instead of concentrating into a highlight — so the eye reads colour rather than reflection, and the colour looks uniform from every viewing position.
Matters most on large wall and ceiling areas, where any highlight would be distracting, and in rooms with a strong directional light source. Matters least on small areas and on surfaces viewed straight on under diffuse light, where the difference between matt and eggshell is barely perceptible.
Choosing matt for a hallway, kitchen or child's bedroom on the strength of how it looks, then finding it cannot be wiped without leaving a burnished patch where the finish has been rubbed smooth.
Practical advice
Matt finishes are commonly chosen for walls and ceilings where a soft, non-reflective look is wanted, but are generally less wipeable than higher-sheen finishes, which matters in high-traffic areas.
Always confirm with a physical sample in the actual finish before committing to a large surface — see methodology.