Thanks for the explanations, now I understand why I needed it.
I got another question about the 13th pin from arduino. According to arduino pin 13th has an built in resistor so I can directly put a LED to pin 13 and ground. When I connect a column on pin 13 does the LEDs shine less bright because there is an extra resistor built in on pin 13(my 220ohm resistor + arduino uno pin 13th resistor?)