does MacBook Pro need a DAC?

Depending on your "receiver", you may do better with USB, HDMI or even ethernet/wifi from your MacBook Pro to your "receiver"... I have used all three from my MacBook Pro to my Yamaha AVR, which in my opinion, sounds much better than analogue headphone audio out from the MacBook Pro... In my case the Yamaha AVR has a nice Burr Brown DAC built in... I have since upgraded to a Lumen U1 Mini streamer and Chord Qutest DAC for yet another level of audio nirvana...

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.

Part of the answer to the OP's question is another question.  What are you listening to using your MacBook Pro as the source?  Music, movies and videos? Another question is whether your receiver and speakers will readily allow you to hear the greater resolution/clarity of a separate external DAC.  

In general, I agree with aka_ca.  You can achieve a significant improvement in sound quality with a modestly priced Audioquest Dragonfly DAC, which plugs into one of your laptop's USB ports. Is it necessary?  No.  But if you listen to music, particularly acoustic music (classical, jazz, acoustic guitar), the improvement in sound quality should be very noticeable. For rock music with lots of electronic amplification, probably less so. For movies, a lot will depend on how much you listen to vs. watch them.  

A number of dealers who sell these external DACs have good return policies, so you can try one and see if the difference in sound is enough to justify your spending the money for it.  

Take a look at a Schiit Modi 3,  excellent performance and inexpensive.  You will hear a difference immediately. 

John Darko compares MacBook DAC with $100 Schitt Modi 3
https://youtu.be/ih0EqxsTBq8