Why BGMApp plays audio in Low-Level layer(Audio HAL ) instead of Mid-Level layer (Audio Unit )?

[visla-jiaozebo]

it's more easy to let the BGMAudioDevice do the play on AudioUnit, but now it's using CAHALAudioDevice which I believe is based on Audio HAL, which is more difficult to code and less robust. so why do you use Audio HAL, what's the advantage or reason to use it.