1. It removes the oxide layer from existing surfaces so the solder sticks and flows better
2. It increases surface tension of the liquid solder so you get a nice round shape and no whiskers and less bridges
3. It helps distribute the heat a bit. It works a little bit like thermal paste, especially when the contact area between the tip and the pad/component is very small.

Generally, flux is your friend and soldering without flux is terrible.

Hange the styles of your cells?