So there are a couple of uses.
First, for the best sound quality, Roon recommends not using the Core to connect to the system. The reason for this is that any processing increases the electrical noise making it to the DAC.
I use the iFi Zen Stream along with a NUC running ROCK as my core. In my opinion, it is an improvement from running the USB from my core to my DAC. Not night and day, but noticeable.
For others who are not using a Nucleus as their core, there is the potential for fan noise to also intrude on the listening experience. Using an endpoint means that you can put it in another room.
It also allows you to have multiple endpoints around the house.
Is the Slack better than a Raspberry Pi?. No idea. Is it easier to setup? Probably
If you are only going to have one listening point, have a Nucleus or fanless NUC, there are probably better things to spend your money on than the Slack or similar.
But it might be worth trying a Pi to see if you see an improvement.