The ridge represents the wear since new. It can cause a squeek against new pads that occurs without braking. The risk of this occuring is the reason we eventually replace the rotors on a good performing system. In other words on my own car I would replace only pads till I had a noise or performance problem (feel, pulse, grab, groan, moan, pull, etc.) and then I would replace the rotors. As a technician I must decide that point before it occurs and we usually replace the discs every second set of front pads.
Bosch Master, ASE Master, L1
33 years MB technician