Safety critical avionics equipment is a prime example of systems that must be tested and retested both mechanically and electrically to help achieve a target zero percent failure rate. Since JTAG/Boundary-scan technology is now commonplace within the components built into these systems it can be used throughout the design's life cycle as part of the rigorous testing regime. In PCBA (printed circuit board assembly) production for example, JTAG is used to (electronically) test the fidelity of interconnects between the mounted electronic devices. What's more since it achieves this by driving signals from deep inside the device or chip (rather than traditional techniques that drive high-currents external to the part) the testing is more harmonious, less harmful to the circuits under test and potentially avoids device longevity issues.
Use of boundary-scan techniques will also reduce the test equipment design burden by reducing the requirements within functional test. In service JTAG/Boundary-scan is also used extensively where programmable devices can have embedded code repaired or 'up-issued' as part of a preventative maintenance schedule.