When driving capacitive loads (i.e. long cables), the output stage might have stability problems, as the phase margin is reduced.
A load isolator has very low impedance at audio frequencies and rising impedance at high freqs, say 200k and up. This will improve the phase margin again and restore "perfect" stability.
You can fake the stability improvement by replacing the load isolator trough a small resistor. But this will cause higher distortion in the output transformer and rise the output impedance at audio frequencies, both usually not wanted.
