Using a DAC with a pre/pro

I use a meridian 568 as a pre/pro and have noticed a few people using a meridian pre in addition to the pro?? I was under the impression that the dac of the processor would handle this capacity? Is there a benefit in using a dac with the digital from cd/dvd/satellite THEN dac to pre/pro?

Thanks in advance
"=>" = L/R analog
"->" = digtal

If you have CD->DAC=>568=>L/R channel amp

You could also have DVD->568=>L/R channel amp

This would use the Processor and DACs in the 568 for DVD use. The 568 internal DAC would be bypassed for CD listening and just use the 568 as a switching device. This would be true "IF" the 568 does not convert the input analog signal to digital and back to analog again.
I understand the physiology of the connections, just if any meridian folks know of an improvement using a dac and bypassing the internal dacs.
I have the meridian 568 as well. All the analog inputs are converted by the 568's internal adc to work in the digital domain. I believe this is mentioned in their website if you look it up. Hope this helps.