does MacBook Pro need a DAC?

Does not require an external DAC, but one can improve sound quality. You may or may not think it’s worth the effort.

All true. Just to expand. a) the MacBook pro has a built in DAC =. b) it is mediocre. c) what is your quality standard? In a reasonably resolving audiophile system, even a $99 Schiit Modi will destroy the internal DAC. A real DAC is a revelation.

Caveat #2: This is about very high quality sound. none of this matters without a good --> to very good system. YMMV.

If money is an issue, you can get Audirvana for $79.00  It is a vast improvement on the built in DAC. 

Either i'm missing something here or need to point out an error.  Audirvana is a media player (software). It is not a hardware digital-to-analog converter.  If you use Audirvana (or itunes or any other player) you still need a DAC to physically create the analog wave-form from the code.  The MBP has a cheap one built in.  You can buy external, better ones from $100 -->> $10,000, with diminishing marginal returns.
Someone also asked about the superdrive.  In general any such removable, optical media, compared to playing the ripped file off the HDD or SSD (better) will generate higher noise at minimum. Whether you find this significant depends on many factors that i wont get into.

quick comment: there is talk here of the headphone jack being optical as well. Maybe, maybe not, either way don;t use it. Toslink is merely the optical version of SPDIF. It has some disadvantages - limited to 96 kHz, and very jittery. Since you have USB, use that instead. Toslink is the last choice for great sound.