Why does an audio DAC possess deemphasis hardware
Historic reasons. 

Part of the "red book" CD specification allowed CDs to be recorded with a specific preemphasis characteristic. When used with a 44.1kHz sample rate, to decode those specific CDs (a minority of all CDs) you need to apply the inverse (deemphasis) characteristic. 

This can be done externally with an R-C network but it's cheaper to do it on chip.

I doubt anyone has bothered with preemphasis/deemphasis in a new digital audio system for about 20 years... which means you can safely ignore it unless you have a very specific need.

Another historic note : When TV stereo sound was digitised to 14 bit and compressed to 10 bit, a more aggressive preemphasis standard (CCITT J-17, boosting high frequencies by 18.75dB) was used (part of the 1980s NICAM-728 standard). In this system, it was required to get acceptable performance at all, as quantisation noise would have been clearly audible after 10 bit compression.

Preemphasis may have been important when CDs were also planned to be 14 bit, but by the time the CD standard settled on 16 bit, it was much less important, and not much used in the end.