Driving LEDs without a resistor:
LEDs are current to light devices not voltage to light. The forward voltage is for reference only.
I just looked through 10 data sheets for parts we are using. Often the forward voltage is only stated as typ. Several LEDs were rated as 3.2V typ, 4.0 max and they did not rate Vmin. As a hobby you do what works. If you purchase millions you will find parts that need 3.7 volts and parts that need 2.8 volts.
LEDS are like a Zener diode. IF 3.0 volts = 10mA them 3.2 volts =20mA. Worse yet the V-forward drops with temperature. As the parts gets hot it will pull more current (in constant voltage move). Runaway condition.