Why do brake pads have a groove?

[Kestas]

Nearly all brake pads I buy now have a vertical groove cut in the middle of the pad. They never used to have that in years past. Why is that?... to channel vapors produced during braking?.... to relieve stresses from volume change during thermal cycling?

 

[technica]

Exactly correct. To release the pressure between the pad and the disc, especially under heavy/severe braking.