There are a couple of reasons to use a preamp in a digital system. One is if you have multiple sources. Another is that some people experience improved sound when using a preamp between their DAC and the amplifier. Finally, some people like their speakers and using digital volume control can result in full volume “mistakes” that can cause speakers to be damaged.
I use a tube-based preamp (actually a line stage) and feel it sounds better than my DAC straight to the amp with digital volume control. I am not sure why but I suspect the line stage matches up better electrically to the tube-based amp than the DAC does.