Why we need additional controller for debugging main controller, instead we can use main controller JTAG?

 

I have no idea because you forgot to mention what hardware, software or system you are debugging.

 

If your PC with debug software has built-in JTAG interface then then you don't need any additional hardware to debug your controller. Usually PCs do not have JTAG interfaces and you must use some circuitry with embedded controllers to convert JTAG signals to PCs USB(for example) interface.