What is the use of the OUT register of GPIO in nRF52?

Question

In the very first blinky example from the nRF52_SDK_0.9.1:

#include "stdint.h"  
 #include "stdint.h"  
 #include "nrf_delay.h"  
 #include "nrf_gpio.h"  
 #include "boards.h"  
 #include "nrf.h"  

const uint8_t leds_list[LEDS_NUMBER] = LEDS_LIST;  

/**
 * @brief Function for application main entry.
 */  
int main(void)  
{  
    // Configure LED-pins as outputs.  
    LEDS_CONFIGURE(LEDS_MASK);  
    // Toggle LEDs.  
    while (true)  
    {  
        for (int i = 0; i < LEDS_NUMBER; i++)  
        {  
            LEDS_INVERT(1 << leds_list[i]);  
            nrf_delay_ms(500);  
        }  
    }  
}

For this one:

LEDS_INVERT(1 << leds_list[i]);

In the board.h you could find the #define here:

#define LEDS_INVERT(leds_mask) do { uint32_t gpio_state = NRF_GPIO->OUT;      \  
                              NRF_GPIO->OUTSET = ((leds_mask) & ~gpio_state); \  
                              NRF_GPIO->OUTCLR = ((leds_mask) & gpio_state); } while (0)

The first line means that, give the value in the OUT register of the GPIO to the variable gpio_state.
You could find the related #define in the nrf52.h and nrf51_to_nrf5.h
And these are the result I print out about the gpio_state value:
gpio_state is: 20060
gpio_state is: 60060
gpio_state is: e0060
gpio_state is: 1e0060
gpio_state is: 1c0060
gpio_state is: 180060
gpio_state is: 100060
gpio_state is: 60
gpio_state is: 20060
gpio_state is: 60060
...

Here is my question: which part of this function changes the value in the OUT register of GPIO? Why it would be change like this way?
Thank you all.

RK · Accepted Answer

The OUTSET and OUTCLR change the values in the OUT register of GPIO. This is a standard pattern repeated all over the nrf51 and nrf52 series, an xxx, an xxxSET and an xxxCLR register to read/write, set and clear bits in the 'xxx' register. this is documented very well in the nRF51 series reference guide. 
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