Digital signals may have one of a number of distinct values whereas ..

Analogue signals may vary continuously within a given range.

Computers use digital signals so any analogue input would need to be converted to digital input before the computer could process it. This can be done by an analogue-to-digital converter.

Sensors in control systems may use analogue measurements (eg Temperature, light, sound) as input to a computer. Sometimes the digital output from a computer needs to be converted back into an analogue signal in a control system.

Speech recognition systems would need to convert sound (analogue) into digital input.

Sound systems where sound is recorded using a microphone, would need to use an A-D Converter to convert the analogue sounds into digital data.