suppose you connected your Wired4S DAC directly to your amplifier (I believe DACs are built with analog output stages that can drive a power amp w/o any electrical issues), how would you control the volume? You would need a preamp OR some sort of attenuator box.
In the bargain, it's possible that your preamp would change the sonics to your liking OR more to your liking.
So, the preamp can potentially have a huge effect on your overall sound.
Traditionally preamps do not have DACs built in because of the general line of thinking that a dedicated chassis for a particular function (pre, DAC, power, CD) can do a better job than putting 2 (or more) functions into 1 chassis. In particular to the DAC + preamp: the DAC is a digital circuit that uses a clock. Clock signals are generally square waves & square waves have a lot high energy (odd) harmonics of the fundamental frequency. These harmonics have the nasty habit of victimizing quieter analog circuits in the preamp. This creates distortion in the preamp output. Great care has to be taken to isolate the power supply to the DAC from the power supply to the preamp. Many other digital circuits also have to be isolated from the analog preamp circuits. To do a really good job by the manuf this increases the cost of the DAC + pre total package.
Down the road, if you like your DAC & want to upgrade the pre (or vice-versa) you are stuck - you have to sell the DAC + pre total package & look for another DAC + pre unit OR buy 2 separate units. Plus, your music system is down - no tunes!! :(
The DAC also influences the overall sonics - There are non-oversampling DACs & oversampling DACs & each has its own signature. There are a great many threads on Audiogon discussing this.....
If you end up in the camp that thinks no preamp is the way to go, you can search for a DAC with a volume control & run that directly into your power amp. There are several such units in the market such as the Benchmark DAC.