Zerotohundred

As the title suggest,
Why does manufacturer apply such system. For example, the olw Merc W124 is equipped with a rear brake disc but the handbrake is using a drum and brake shoe. It looks like any normal disc because we cant see the mechanism on the inside center hub of the disc.
Other vehicle using is is the Pajero Sport.

Some conti cars are using seperate smaller caliper on the rear disc as handbrake.

Why do they do this?

I read online someone said its to prevent the disc from being...