JTAG is a standard interface which is not true for dedicated debug interfaces found on most MCU's. Due to this standard interface its possible for other vendor's to design tools.
Because of the JTAG support in ARM, we can have ICE boxes (In Circuit Emulators) or Trace boxes from other vendors and connect o ARM. The debugger on the host system can then use the drivers that comes with the ICE box to connect to the ARM. This helps a lot in debugging efforts.