Short answer
While you can project directly onto a flat white wall, using a dedicated projector screen significantly elevates picture quality.
Why Walls Fall Short
Surface Texture Impairments: Standard drywall textures (such as orange peel or knock-down finishes) cast micro-shadows under bright projection light, breaking up fine 1080p/4K detail.
Color Uniformity: Standard household wall paint contains color tints that skew color accuracy and white balance.
Gain & Black Level Loss: White paint scatters light diffusely in all directions, washing out contrast and causing reflections to bounce around the room.
Popular Screen Types
Standard Matte White (Gain 1.0): Evenly distributes light in dark rooms for wide viewing angles.
Ambient Light Rejecting (ALR): Uses microscopic angular optical layers to reflect ambient side-light away while bouncing projector light directly toward viewers. Crucial for living rooms with lights on.
Ultra Short Throw (UST) Screens: Features microscopic triangular sawtooth structures designed specifically to reflect light thrown from steep angles beneath the display.