Why do we need control the led current?

We can provide answer if we know that voltage-current characteristics of led.
In fact, the LED is kind of diode(it’s efficient Silicon Light-emitting pn Diode Prepared by Ion Implantation Locally-doped Defects) , it has voltage-current characteristics. the both sides of current of led PN junction is nonlinear . other work is: the led PN junction have slightly been increased will causing a sharp maximum in the both sides of led.  the current will reach(or exceed) the limit value and damaging the led.  the control current is necessary when using led.