A brake pad is more than just the friction block. Bonded to the back of that friction material is a rigid steel backing plate, and it plays several roles in how the pad works. Here is what the backing plate does.
It carries the friction material
The backing plate is the steel spine the friction compound is bonded or riveted to. It gives the pad rigidity so the caliper piston can press the whole face evenly against the rotor, rather than the friction material flexing or crumbling.
It transfers the clamping force
The caliper piston pushes on the backing plate, and the plate spreads that force across the friction material and into the rotor. The plate ears also locate the pad in the caliper bracket, riding in the abutment clips so the pad slides squarely.
Why rust on the plate is a problem
Corrosion can build between the friction material and the plate (delamination) or on the plate ears. Rust on the ears makes the pad bind in the bracket - a common cause of drag and noise; see why brakes squeak. Severe delamination can shed the friction chunk entirely. Quality pads use coatings and mechanical bonding to resist this.
What to look for
When servicing, check that the friction material is solidly bonded and the plate ears are clean - see brake pad wear patterns. Fit quality pads with good backing plates and fresh hardware.