I believe that this adds the Repository to the Source, which causes it to be then inherited by the Source Citation.
However, for me, having the Repository be editable as part of the Source Citation and not the Source seems to make more sense. As an example, imagine that I have multiple Birth Records from Ireland. I have collected them from different sources:
Family History Library, SLC
General Register Office, Dublin
My inclination is to create a single source (Vital Records; Births; Ireland). Then on the Source-Citation, select the appropriate Repository.
Maybe I just need to change my thinking, and create 4 unique Sources (each with their own Repository).
Maybe I'm just rambling.