Many metallic catalysts, particularly the precious-metal ones, are often deposited as very thin films on a substance

Question:

Many metallic catalysts, particularly the precious-metal ones, are often deposited as very thin films on a substance of high surface area per unit mass, such as alumina (Al2O3) or silica (SiO2).
(a) Why is this an effective way of utilizing the catalyst material compared to having powdered metals?
(b) How does the surface area affect the rate of reaction?