Not quite. You are correct in that It is can be a used as a pre amp but it can also be used a as headphone amp and has enough power to drive them for a quality sound. Though it's primary function is a external dac
Typically, headphone amps are small and very low in power and not enough to drive conventional speakers. The DM's, Beresford and other similar dacs can also be seen (depending how you look at it) as headphone amps with a quality dac and pre amp functionality. I used to have a Beresford tc7510 dac which, for test purposes, I used as pre amp and as headphone amp for a while. Couldn't fault it one bit
For more info and better explantion than I can give have a look at the following link.
Do my Headphones Need a Headphone Amp? | headphone.com
Some CD players have head phones sockets and can be used without an amp.