Zero-latency filters (typically IIR designs) process audio with delays measured in microseconds — far below human perception. This makes them suitable for: live performance monitoring, real-time effects processing, and any application where the performer must hear the processed signal without delay.
The trade-off: IIR/zero-latency filters cannot achieve linear phase. They introduce frequency-dependent phase shift, which can subtly alter transient timing and stereo imaging. For critical mastering applications where phase linearity matters, FIR filters (which add latency) are preferred. For live/realtime use, the imperceptible delay of IIR filters is the clear choice.