Why do I need an additional player device if I have a Roon nucleus and a dac?

Based on the reviews I have read, I do not consider the Nucleus overly expensive for what you get, and the build quality.  If I ever needed to replace my Antipodes DX I would not hesitate to consider a Nucleus Plus with an internal or external drive.

Regarding, endpoints, the Nucleus can serve as both renderer and endpoint, as can my DX, and that will be great for many listeners.  However, in addition to the reasons given above, the reason I use a separate endpoint is sound quality.   I use Ethernet to connect my server to my endpoint and then use I2S to my DAC.  Between my server and endpoint, I use two TP Link optical converters to optically decouple the Ethernet connection.  The end result sounds better to me than running the whole thing through the DX only.

You can for sure use JRivers26, Almarro or others as well as Qabuzz, Roon with your pc .what I do highly recommend and truly an upgrade is the Jcat femto usb card, as well as femto ethernet 
card a world of difference ,for not only uses the best commercial femto clocks but regulators and isolation from the computer noise
from the musical signal .easy to install and a highly recommended.
just see excellent reviews .

A Network Acoustics ENO filter on the Ethernet input of the streamer makes a big improvement too.

It is amazing how many people need a course in manners.
emergingsoul, I understood exactly what you you were trying to say. You also could build a NUC instead of the Nucleus for less than 1/2 the price.

Esoul.  you are understandably a little confused about how the roon stuff comes together.  you are not alone.
This youtube is a great help:  http://https//www.youtube.com/watch?v=-ZVcNx-DlIc&t=7s
John Darko does a very good job of breaking it down in all sorts of configurations.