I think it's about the output voltage of the DAC and the input voltage of the amp. I plug my Orchid DAC directly into the Decware amp. I control the volume through software and it sounds good to me. The Decware amp has its own volume control so I keep trying to find the sweet spot though. the biggest issue for me is that the output of the flac files vary so much I can't just find a volume level and leave it for multiple albums.
The preamp can either attenuate or boost the DAC signal if its powered or just attenuate it if it's passive.
Using a preamp also would add tone controls if you use them.