What are the advantages? Thanks.
Yeah, probably none.
Almost every manufacturer (of everything, not just audio) touts their design decisions/trade-offs as features or advantages.
I've never heard a difference/defect from a DAC, including the one that was built-into my 1st CD player. (I did have a noisy soundcard once that made noise when the hard drive was accessed, but I don't blame that on the "DAC".)
What does a 6 channel headphone amplifier do?
You can plug-in 6 headphones at once.
I assume the firmware allows you to adjust the volume individually but knobs would be more convenient.
Multi-output headphone amps are common in recording studios, radio stations, and other professional situations.
I also see multiple connector types so you don't need adapters to plug-in a headphone with different connectors.