While both Botox or Dysport and fillers are used, for example, to address aging of the face, they work in completely different ways, and therefore are used very differently. In fact, simply swapping the two could produce very undesirable results.
A basic, simple explanation:
Botox is injected into the muscle (in the face there are many) to paralyze that muscle; you can imagine that injecting it into the wrong place could be a problem! In the right place, it is effective and safe.
Fillers are essentially injected into the wrinkle, line, or fold. Adding volume is the goal, and that's how you see results.
To achieve ideal results for my patients, I often use both products as part of my palette!