Why optocoupler (4N35) needs resistor?

Question

I am learning how optocoupler works. Now I am passing Lesson 8 4N35

I've found why resistor needed for LEDs, but why there is need for resistor between optocoupler and power supply/input pin?

Answer 1

Inside the optocoupler is an IR LED on one side (between pins 1 and 2) in the image below and a phototransistor on the other (pins 4, 5 and 6).

And so the resistor is there for exactly the same reason it is in every other LED circuit to limit the current and thereby protect the LED, as this quote from the tutorial you are following makes clear:

To protect the LED from burning, usually a resistor (about 1K) is connected to pin 1.

These two youtube videos do a good job of describing how an optocoupler works:

How an Optocoupler (Optoisolator) Works

Using Optocouplers with the Raspberry Pi

Note: that while the too optocouplers used in the above videos are different (and have a different number of pins) the functionality and operation is the same.