Realtime group scheduling doesn't know JACK

Recording musicians. If there's no way of monitoring what is being played direct from hardware (which is sometimes the case) there needs to be minimal latency between a musician playing something and hearing it back from the recording system, otherwise the musicians can't play in time with each other. If you're paying top money for studio time and session musicians, what might be near enough for a "Guitar Hero" type game isn't good enough. A 20ms delay is enough to cause a problem (musicians don't tend to stand 20 feet from each other in a studio).

There's also the situation when recording or mixing a lot of tracks, possibly with lots of DSP going on, you need certain things to happen within a short deadline to avoid audible dropouts. Large buffers help, but if your system decides to put audio on hold while it does something else it's pretty inconvenient (and in professional audio, inconvenience = cost).