This allows LCDs to operate at low power, as no energy needs to be spent generating light.
To use ambient light, the liquid crystal itself is sandwiched between a polarization filter and a reflective surface.
The mirror makes the display opaque so it cannot be illuminated from the back.
The monochromatic light from an electroluminescent source does not work well with color displays, however.
An incandescent frontlight was therefore a popular accessory for the Nintendo's Game Boy line up until the first model of the Game Boy Advance; the Game Boy Advance SP introduced a frontlight, which made external lights redundant.