Server/Streamer or Streamer/DAC?

What is it about the more expensive streamers that provide better quality sonics? Anyone want to tackle that one? 

I am in the process of considering adding a streamer to replace my Macbook (via usb) direct to my DAC. I just want to get to the bottom of the sonic improvement I'd be getting. I have a high end DAC (Rockna) and would add their streamer or something at that level. I just don't understand why, from a tech perspective, a high end streamer is better than a computer straight in via USB. 

JC5= I am kind of in your same boat.  I have a Lumin X-1 which is a DAC / Streamer and I love it.  However, I have been debating whether to add a music server to even improve the sonics more.  Is that guaranteed????  I don't know.  A dealer wants me to buy the Rockna Reference DAC and Server.  I was also considering getting an ultra high end DAC like Lampizator Pacific or Aries Cerat Kassandra or Helen.   I wonder how much better the sound would be?   From a  server perspective,  I have looked at Innuous, 432 AEON, Taiko, Antipodes etc.  I would have to sell my Lumin X-1 before I do anything and I am sitting here now listening to music and saying OMG this sounds so good.  But I am intrigues if a server will take my system to another level.  

What is it about the more expensive streamers that provide better quality sonics? 

Purpose built to optimize audio quality with higher-quality parts while minimizing contamination and noise. 

There is a fair explanation of the benefit of using a better server/streamer, as compared to something like a stock computer, on the Antipodes Audio website and the reasons why. A soft tech explanation which enlightens.
https://antipodes.audio/antipodes-technology.php

I mostly play from a SSD, but occasionally stream from an unoptimized feed - stock modem, generic cat 5, via WiFi - into my Antipodes CX+EX. The CX does clean things up a bit. The empirical result is an SSD is much better. The stream is a little murky, less dynamic, less resolved, and lacks tone color in comparison. However, it is still good enough to be enjoyable. Optimizing the stream is on my to do list. Many report excellent results with audio grade switches, LPS, cabling, and other gadgets to isolate and eliminate noise.

IMO, you cannot overstate the importance of feeding your dac with a quality signal. There are many good, if varied, dacs out there. Servers/streamers are going through a phase of rapid progress and the SQ delta is large over a small time delta. Spend more on a streamer than a dac. The opposite was true 10 years ago.

Current digital front end: CX+EX > Ideon 3R MT reclocker > Lampizator Pacific.

I’ve ordered a Lucas Domansky Music Server to replace the CX+EX. Maybe a Taiko Extreme eventually. The CX +EX is very good, but the ball keeps rolling.

Streamers improve on computers by better and multiple power supplies, better USB ports and software as well as lots of measures to suppress EMI/RFI that computers tend to not much care about.
The resulting SQ is in a different zip code altogether. 
There are though people on this forum that believe that a good DAC will through its noise rejection deal with all the issues streamers are trying to solve, I for one have never encountered such a DAC.