JTAG was created to test populated circuit boards after manufacture. Nowadays, JTAG is primarilly used to access sub-blocks of integrated circuits and useful mechanism for debugging embedded systems.
When used as a debugging tool, an in-circuit emulator - which in turn uses JTAG as the transport mechanism - enables a programmer to access an on-chip debug module which is integrated into the CPU, via the JTAG interface. The debug module enables the programmer to debug the software of an embedded system.
Besides debugging, the second purpose of the JTAG interface is allowing device programmer hardware to transfer data into internal non-volatile device memory. Some device programmers serve a double purpose for programming as well as debugging the device. [Source: http://en.wikipedia.org]