Does PCI-E Sound Card improve sound quality vs onboard Optical out

I found it difficult to perform a proper search for this question so please bear with me. I run optical out from my onboard audio to my receiver. If I add an internal sound card and only run optical-out from that to the receiver will that improve audio quality over the onboard optical out? I have heard it both ways. If it does then I wonder how. My understanding was that the onboard audio just passed through the optical-out to the receiver for processing. I guess that it just my assumption. Help is appreciated. ---Chikn