Why modern day microcontroller have a JTAG port?

JTAG helps in debugging of embedded software for micro-controllers. Also it is IEEE standard and thus makes it easier to develop debugger tools. Jtag also eliminates the need for in-circuit emulators which are more costlier option. JTAG port can be used to push instruction into a micro controller when micro-controller is not able to fetch instruction from memory and thus aid testing and debugging. JTAG can also help in reading the state of internal registers even though micro-controller core is halted.