Here is an example of two materials being used because they best suit the intended purpose. In this case high grade marine ply and very thick MDF:
The marine ply piece is serving as a brace. The thin curved piece needs to be strong and accept screws along its edge. No problem with marine ply. MDF would not have been as good a choice.
The MDF piece has a smooth curved edge which will later be finished to a very high gloss, painted finish. I could have used plywood for this, but from experience the exposed laminated edge when routed is harder to get a good finish on. Again from experience the laminated edge over time can subtly change with changes in temperature, expansion, contraction, and so on leaving a slight distortion in the high gloss finish i.e. a visible ripple. The MDF will have no such issue, ever.
Used properly, MDF has properties that make it superior to other materials in certain circumstances.