Delay is ubiquitous in the natural world. There is a delay when sound reaches one ear before the other. Sound bouncing off buildings and walls is delayed. There is a delay when you listen to music from a loudspeaker which is further away than another. Multiple sounds reach your ears at different times and overlap with each other.
The simplest delay effect is an echo. Overlapping echoes produce comb filtering. Comb filtered sound with a variable delay time results in a chorus effect and flanging. Overlapping echoes with variable delay times and amplitudes produces reverberation.
Delay is usually thought of as a time-based effect. However, time and space are linked. A delay in time is equivalent to a distance in space and vice versa. Sound in air travels at a speed of about 1100 feet per second. This means that a sound that has travelled a distance of just under one foot is equivalent to a delay of one millisecond. Consequently, delay is used as a sound effect to emphasise either motion in time or motion in space.