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.
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.