Why buy Bluesound Node2i vs. Marantz AVR for playing music from NAS or streaming services?

Yes, you will be well served by outputting to a higher quality DAC when you use the Bluesound Node.
I have 2 Node's and output to a pair of Ayre Codex's.
The Ayre's are definitely higher resolution than the Node onboard DAC (which is no slouch for the money).
You can score a used Codex for a little over $1K.
Otherwise, I would consider a Schiit Gungnir or Yggy, both are under $1K used, and like the Ayre can be upgraded when upgrades are available.
Bob

I use the node 2i to very good results into Yiggy 2.
Change out the power cable and use a good coax SPDiF cable you be rewarded.
You will not find a used Yiggy 1 or 2 for less than 1600.

What power cable and digital cable do you use 

Basic power cable. 

Audioquest Carbon Optical from Node2i to Parasound

btw for external DAC, which is a better fit for my system - Qutest, Yggy, Benchmark DAC3, Ayre Codex?  I listen to all kinds of music incl classical, jazz, rock, pop, vocal 

Because the Vault 2i is awesome.... it has completely changed the way I listen to music.  Not only is it great in my main system, I can send its content to my Blutooth speakers in bathroom and garage.  Best money ever spent on a component with respect to what you get for your money in one box.  I am actually listening to it right now...at 430 AM...

I send its digital output to a DAC but it sounds pretty good through it's analog out.  It has a combo digital optical / analog inputand a sub out.  You could literally drive a pair of powered speakers and sub.  I usethe analog input for my tuner.  My pre has no remote volume so at night I just use the Vault's volume.  

It can work with a number of music services.  It has bi directional Blutooth, it can receive content from a phone or tablet.  Want a family member to stream to your stereo without physically touching it?  Just have them putrhe Blu OS app on their device and they can stream their contentto the Vault 2i.  The Vault is worth the $$$ in my opinion.  

The Bluesound will give you a lot more options for streaming - Qobuz, Tidal, Radio Paradise, and much more.  

Connecting to an external DAC will improve sound quality, but the DAC in the Bluesound is quite good for the price.

You should notice a difference in sound quality running the Bluesound and external DAC through your Parasound gear over using the amp/DAC in your Marantz receiver.  

The Bluesound app is very good and makes it easy to use your phone to select music from either your NAS or from streaming services.

I use to use the Bluesound Node and it was excellent (I no longer need it since I have a Roon core direct to my Dac). I think you'll be surprised how good the Bluesound DAC performs. One great thing about the Node is you can run both digital out and analog outs at the same time. If you ever need to MQA, you can just run the bluesound if your external dac does not have the capability.