RVM: rosin core solder is used for electronics and if you were soldering on a printed circuit board you would not want to use extra flux. The flux in the center of the solder would be adequate.

But, when working on other than PCB's it is very much a help to add external flux as well. I use external flux, for example, when I build battery packs. And when I solder the connectors for the battery packs. And of course, for any mechanical type soldering like landing gear wires. You will find that using an external flux just helps the solder to flow faster and so you do not need to keep the iron on the work for as along. This is important if you are soldering battery packs together because you do not want to overheat the batteries. On other types of soldering, it is just a nice convenience.