Preamps without built in DAC silly question

Forgive me for posing such a silly question oh great and knowledgable ones:

I am considering picking up an Audio Research REF 5 SE. However, i do not see any digital inputs where i could go straight from my media PC to the pre-amp. Therefore i will have to keep using my Wyred DAC and connect to the AR 5SE via my balanced interconnects then out to my Levinson 331 Amp. If the DAC unit is doing the conversion already, why would one still want a pre-amp in-between the source signal and amplifier? What exactly is the pre-amp doing if it is not needed for the digital to analog conversion. Will the preamp "improve" the already converted signal? I know i must be missing something very obvious here so please enlighten me. If my logic is correct (which i doubt), the pre-amp would not make any sonic difference. Hence to really change my sound, wouldn't i need to also change my DAC? I think i'd be going backwards if i were to let my media PC do the DAC duty and send signals via RCA to the AR Ref5se. Lastly, besides making more products to sell, why would a pre-amp not include a DAC unit? I'm guessing its to keep prices down or keep things simple in the pre-amp? I look fwd to your replies.

The audio research DSPre ha a built in DAC. Personally I would never buy one. I don't want to have to change my preamp if I want to upgrade my DAC.

The preamp is the control center of your system. A volume control on a DAC just can't compare to a truly great preamp.

I would suggest you borrow a Ref 5se from your dealer and listen to it in your system. Try it both ways and see which way you prefer. That is the only way that you will know for sure.
Reasons to use a pre-amp:

1) you always need a volume control somewhere and no volume control otherwise
2) you may need to switch between multiple inputs
3)pre-amps tend to impart a particular sound quality that one might like. This is more of value when you have multiple sources and you want them to sound more similar.
4) there may be other useful features in teh pre-amp like balance and tone controls among others.

That's pretty much it. Pre-amps are optional in more cases these days especially if one has a single digital source with a decent volume control already.
Mapman pretty much nailed it. If you do not need input switching, a DAC with a very good volume attenuator is probably the cleanest, most honest sound you'll get out of that source.

If your Wyred DAC - you did not state the model - has a good volume control, I'd say give it a whirl direct-connected, you may be pleasantly surprised!

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.
My DAC is the wyred DAC2. Sound i get is nice but as with this illness of ours, im constantly looking for improvement or different sound. Ive never tried tubes in my audio system but do prefer tubes in my guitar amp. The volume control is nice and i dont intend on using any other input devices. Hence 8f i can bypass the preamp and save some money, i can put those funds towards ref250 vs ref150 or something else.
A pre-amp is designed to drive the input of a power amp. A DAC may not possess this capability, the design intending for it to be connected to only a high-impedance pre-amp input (the higher the better. Some power amps have a low impedance input). There is also the matter of the input sensitivity of your power amp. Your DAC may or may not have enough output to drive the power amp to full output (another job of the pre-amp). I would contact Wyred and ask them which is true of your DAC. Some people not needing any gain from a pre-amp, only attenuation (volume control) and impedance matching, choose to go with a passive pre-amp (no active circuitry), finding passive more transparent than active.
I built a DAC into my tube preamp and it's the best sounding combo I've heard. Very short signal path this way, one less set of ICs, and the preamp output tube also functions as the DAC output stage. Switching is set up with high quality relays so that when the unit is not being used as a tube output DAC, the DAC and its power supply are completely turned off, so no digital demons can get into the rest of the circuit.

So, when it's being used as a DAC, it is a tube output DAC with SPDIF and USB inputs directly feeding my amps (no “additional“ preamp in the chain) and otherwise with the flick of a single switch it is a tube preamp with 6 analog inputs and no additional digital nasties present, since the entire DAC circuit including power supply is powered down.

I'm surprised commercial manufacturers don't do it this way. More complex to build but a “best of both worlds“ outcome.
In case you're curious, a pic of my preamp/DAC. It uses balloon 26 DHT output tubes; the four tubes in front are voltage regulator tubes.

Max26 Preamp/DAC