It helps eliminate brake fade due to trapped brake gasses and debris caused by pad and rotor wear, hence, allowing debris to escape and not become part of the pad rendering it noisy and pitted etc, but mostly allows for expansion due to heat which prevents the pads from cracking.
For heavy applications, I will slot two groves with a hack saw for each pad, but normally one slot vertically or diagonally does the trick.
The chamfer on the ends helps reduce noise and add better engagement.
So sometimes, you can cheap out a bit on a set of pads and modify yourself cus if the company didn't do that, they can sell them cheaper since they didn't have to go through the extra finishing details.
But usually a high end pad has better bonding glue, adds in new hardware and some grease and normally has quality pad noise reducing shims already riveted or glued to the back side.
If you buy a set of pads and it has no hardware, no shims, you can bet those pads will suck large from the start.
I got the rotors, there is a difference in weight and feel to them. They feel much more heavy duty and sturdier than ac delco rotors (sound strange but sometimes you can just feel a difference). And the pads already have 2 deep grooves in them. Ill get them on in the next few days and see how they do, im not expecting much change as i dont drive this car like i stole it. Im just hoping the shake will be gone and stay gone for more than 1.5 years.