AdrianAll music these days is digitally stored which means a DAC is somewhere in the chain if you can actually get sound to be produced from said files. It doesn't matter what you plug RCA jacks into (they are solely analog), whether it's a cell phone, laptop, DAP, computer motherboard or sound card, there is a DAC being used in there somewhere. By using RCA "IN" instead of USB you are simply choosing to use that other DAC rather than the one that is built in.
AdrianThis is not true. RCA is merely the connector, which is occasionally used for digital audio. Just lookup coaxial digital audio.
However, using a set of RCA inputs for both analog and digital sources is difficult. In this case, the pair of RCAs is, as you mentioned, for analog audio sources only.