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

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.

Thanks for Darko link. Seems he is using roon to connect to streamers.  A way of promoting products he may be getting paid for.

he doesn’t choose to keep it simple by using nucleus connected to a standalone dac.  His setups are abit involved.  I like listening to him but always wonder about independence.