The reason is that this way we have the switch right at the back of the power supply, thus avoiding interference in the audio that would occur if we ran the mains voltage from the back to the switch on the front and back to the power supply.
For some users in special recording situations, a completely latency-free monitoring is very important. The internal headphone amplifier is useful e.g. for ensemble recordings. You can set up and finetune the main stereo pair or the position of spot microphones without the irritation caused by the latency of digital signal chains.
When recording a singer for a multitrack recording, latency free monitoring can be provided by using channel 1 for the vocal microphone and routing the DAW playback signal to the DI input of channel 2. Selecting the mono option, both signals appear in the headphone’s stereo center.
It could also be helpful to be able to listen to the pure microphone signal in order to check the influences of the subsequent signal chain.