Flat paints provide even light reflection and give the surface a softer, more uniform look.
They don’t highlight surface imperfections like higher sheen paints, so they usually don’t require as much surface preparation (like patching, texturing or sanding) before painting.
They are much less likely to show “lap marks”, so they are generally easier to apply and touch up than higher sheen paints. (When one painted section dries before the next section is painted, the two sections won’t flow together into a uniform film, resulting in a “lap mark.”)
Flat paints don’t need to be sanded or de-glossed before repainting, unlike higher sheen paints.