Drive the LED
This article investigates and compares different way for LED. They were Constant voltage and current limiting resistor.

We are know that the led VF has dropped When the temperature of LED is raised, If this time, with Constant voltage source to drive the LED as time continuous work current will be bigger .. If you use the resistor to limiting current ,right now in the circuit the current is not changed basically(because the resistor also share a part of the currently and temperature).