Doing vertex paint and weight paint is a required steps in creating clothing CC.
So what are vertex paint and weight paint for?
Vertex paint is related to morphing. You do morph when using sliders to make the sims body fatter, skinnier, change the butt size, etc. The vertex paint on your cloth/object CC will dictate how they will be affected.
For example, a puffy skirt that covers legs should not be affected by sliders that adjust thickness of calves, right? If you will vertex paint the skirt with a color that deactivates robe morph, it will not be affected by these sliders. It will also move in a more appropriate for a skirt manner.
Weight paint is related to rigging (beside breast bones, they are also used for morphing)
A rig is a bone structure of a 3d model that enable your model to move. It's like the bones in your body. The weight paint on your outfit mesh is to ensure it get affected by each bones the right way.
Along the process of making clothing cc you will hear about vertex paint and weight paint transfer. Basically, you transfer these data information from preexisting cloth/object of sims 4 to your custom ones, instead of making it by yourself. Of course in certain condition you can adjust it by yourself.