The necessity of DTV measurement is Brake Roughness (Judder)

Brake rotor thickness is varied as increased mileage like the picture.

 

The reason of the DTV value increasing as per mileage is mounted

Lateral Run-Out (LRO).

Even if the DTV value of new rotor is within its allowable specification,

it is apt to wear out not evenly due to the mounted LRO.
As you know, when the rotor mount on an axle it contact with hub flange and

it is supported by bearing and caliper assembled.
In this mounting match, there is some tolerance and it bring un-even contact

between rotor and pad when run or braking with road states.

So the some area of friction part where more contacting

wear out more than other less contacting spot.

By the way the thickness where more contacting

part become thinner as the photo indicated.

Although there are many other factors what bring brake roughness by system interaction

like caliper, friction material, mounted rotor run out and assembly state.
DTV (Disc Thickness Variation) is major cause of roughness over the other factors in turn,

it generate Brake Torque Variation (BTV) and BTV produce vehicle vibration by means

of friction couple that resulting in frictional vibration.
It also generates resonance in the brake components and or vehicle body.

With the above reason the occupants feel roughness through steering wheel,

brake pedal, seat and chassis.

They say, Brake Roughness (Judder) together with brake squeal are major field

customer complaints that almost reach 70% of complaints.