What do you mean? Class D amps are essentially PWM driven amplifiers. Essentially any speaker in existence offers enough low pass filtering to be directly wired to the amplifier, as well.
Imagine if you did dithering to produce grayscale images with a black and white screen by just using different sizes of rectangles; you'd have to have a very high resolution screen before you wouldn't be able to see the individual rectangles. That's what pulse width modulation is like. Whereas pulse density modulation, using a sigma-delta modulator, is more like a proper dithering algorithm, where you spread the pixels out evenly across space, you don't just use different size rectangles to represent different shades of gray.