Individual LEDs as semiconductor elements always require a stabilising current, i.e. the current flow through the LED must be limited at a defined level (operating current). In the simplest case, this is a series resistor aligned to the forwards voltage.
If the operating current is not limited, the LED can destroy itself due to overheating. The forwards voltage reduces when the LED heats up. This reduces the resistance and the stronger current flows, which leads to a further reduction of the voltage and increased heating. If you do not restrict the power supply, the LED overheats due to the increasing current.