Design For Test ensures that the device or product can be tested after the manufacturing process.
Products that can't readily be tested can't readily be modified or upgraded. It can be very difficult to add functional tests if the system was not designed with testability in mind from the onset, the testing "hooks" should be implemented during the design phase not after words.
Good DFT techniques aid fast and comprehensive automated testing that catch manufacturing defects that could easily be missed with less strenuous testing. The end result is fewer defective units shipped.