Bias
Took me a little while to recognize that the current mirror was just a current source for the differential-amplifier. I agree with Ron, this entire circuit is just a basic differential amplifier.
Differential amplifiers are better than regular amplifiers because
-they reject noise better because they reject common signals on the input lines (and if the input lines are very close to each other, the noise present on both lines will be almost identical)
-they do not require you have to have large filtering capacitors on the amplifier output to filter out the DC-biasing which is a big plus when designing on silicon
I find that a bit odd that you knew about current mirrors and DACs without knowing about differential amplifiers. At least for me, I learned about diff-amps and a year before I learned about current mirrors.