When a LED is placed in a circuit and voltage is applied, a voltage develops across the LED called the CHARACTERISTIC VOLTAGE DROP and this voltage depends on how the LED is manufactured as well as the colour of the LED.
This voltage cannot be altered and is a characteristic that comes with the LED.
If you supply this exact voltage to the LED, everything will be ok. This exact voltage occurs when the current though the LED is 17mA for most LEDs but can be 1mA or as high as 300mA, depending on how the LED is manufactured.
If you reduce the voltage slightly, the brightness will decrease but if you increase the voltage slightly, the LED may or will be damaged.
Rather than go through all the complexities of delivering an exact voltage to the LED, it is much easier to add a resistor and allow the current to drop slightly when the voltage drops.
Also, if the voltage increases slightly, the current though the LED will increase slightly, whereas if the LED is connected directly, the current will increase considerably.
This is the simple answer.