Jitter/timing. (Which can be measured, by the way, even if there isn't perfect consensus on what constitutes a 'good enough' result.)
Even if you transmit the voltage levels representing 1s and 0s in the proper sequence 100% of the time, that alone doesn't guarantee a perfect output, as those voltage levels have to occur at precise times. Conducted and radiated emissions are known to be able to impact nearby electronics in undesirable ways, and that can include impacting the multitude of clocks commonly found in digital gear, or downstream electronic interfaces, etc.. Other scenarios are possible as well.
Such emissions are possible on all wires, including those inside a DAC, etc.. One hopes that better DACs have improved wire routing, better internal ground paths, better inherent EMI/RFI rejection, etc., so as to be less susceptible to such influences. Though whether you notice the influence of such effects may very well depend on where you live... the person living in the middle of nowhere may never have to worry about such issues with any reasonably well designed set of gear, while the person living next to the local broadcasting tower array may find that just moving wires causes a change in behavior.
No part of your audio system exists in isolation - it has to be considered as a whole with an understanding as to how individual elements may impact other elements, including across time, across a variety of inputs (music selection), and across various environments. Just because a behavior isn't evident in your environment doesn't meant that it isn't evident elsewhere.
(And that's aside from rather foundational assumptions of equal hearing ability, equally well implemented room acoustic treatments, etc., much less differences in musical or system presentation preferences.)
