When playing a software instrument via an external hardware controller (typically via MIDI over USB), even a tiny delay from the musician striking a key or a drum pad to the resulting audio out of the speaker can become very problematic for instruments with a very fast attack, where exact timing matters a lot to the performance. Examples include drums and percussion, pianos, many plucked type of sounds. With slow attack sounds like legato strings, extra latency is generally much less of a problem.