LED lighting applications combine many LED devices into a single lighting fixture. Most engineers consider LEDs to be fundamentally current controlled devices. As such, it is natural to configure the LEDs into series strings. A typical string will contain anywhere from 5 to 20 LEDs. By controlling a single current, the LED driver can control the brightness of the entire string.  Having a common current in all of the LEDs assures better control of the total string and more uniform LED to LED brightness.