Pull a new brake pad out of the box and you will notice the friction surface is not a plain rectangle. The edges are beveled and there is often a slot cut across the middle. These chamfers and slots are deliberate engineering features - here is what each one does.
What chamfers do
A chamfer is the angled bevel ground into the leading and trailing edges of the friction material. Its main job is to reduce brake noise. Squeal is caused by high-frequency vibration between the pad and rotor; chamfering the edges changes the pad's contact geometry and stiffness so it is far less likely to resonate and sing. It also helps the pad seat evenly during break-in - see how to bed in brake pads.
What the center slot does
The slot cut across the face of the pad also fights noise and vibration by splitting the friction surface into two smaller sections, again shifting the resonant frequency away from the audible squeal range. The slot gives gases, dust, and water an escape path from between the pad and rotor, which helps keep the bite consistent. This is why the slots matter for wet-weather feel.
Do they wear away?
Yes - chamfers and slots are shallow and disappear as the pad wears down, which is normal and expected. They do their most important work during break-in and the early life of the pad, when a fresh pad is most prone to noise. A pad that has worn past its slot is getting thin - check it against pad replacement guidelines.
Why quality pads have them
Cheap pads sometimes skip chamfers and slots to save a machining step, which is one reason bargain pads squeal. Quality ceramic brake pads come properly chamfered and slotted, and pairing them with fresh hardware and shims gives the quietest result. If your pads squeal, see why brakes squeak.