The USB to TTL cable enables microcontrollers to communicate with a PC using serial communications.  Not only does this greatly simply the embedded software (microcontroller firmware) it also reduces the complexity at the PC end; there is no need for drivers to be developed.  The drivers that are supplied with the USB to TTL cable create a 'virtual com port' on the PC which appears as a extra serial port.  The PC end can therefore use existing software that communicates with the serial port such as Windows Hyper-Terminal, or applications can be developed using the standard serial port interface.