Many computers don't use the best DAC in their sound cards (DAC is to translate digital data into analog audio signals), nor do they tend to use the best opamps to drive the headphones, so CD and DVD sound is adequate, but not exceptional.
So, and outboard box with a better DAC/AMP can improve the sound, and almost ALL computers have USB, but only a few have optical or coax out (Macbook has optical). Digital data can come from USB, optical, Coax, and a couple of others. So, many tiny amps that have a DAC include only USB input for DAC to be used with laptops and keep the amp small and battery life long (optical kills battery life).
The DAC converts the digital sound data stream to analog, and feeds it to the amplifier that comes with the DAC via an internal connection. You still get an analog input to feed an external analog signal into the amp, from a headphone jack or line out from a MP3 player, CD player, TV, etc...
If you already have a better portable DAC (Headstage USB DAC CABLE, Behringer, M-Audio, TBBAM, Monica, etc...) then you don't need one in your amp, and a Hornet or P2 or any other normal headphone amp will still sound better than the line out of the laptop. My P2 with Headstage DAC sounds tremendous, and is only a little bigger than the predator, and total cost shipped was under $200.