I have found, over time, that rosin core solder by itself doesn't do nearly as well as a separate application of flux.

Flux is NOT to clean the contacts. I don't know how it does it, but flux does something chemically that allows the solder to really bond to the surface of the metal in a way that just heat alone won't.

Without flux, your solder will have a tendency to just sit on the surface of the metal in a self-sticky blob that can be picked off with your fingernail after it cools. With flux, the solder flows outward smoothly across the metal and gets into all the nooks and crannies.