Volatile function argument in C (STM32F4 example)

Question

Reading STM32 examples I've met code that cannot understand.

/**
  * @brief  This function writes a data buffer in flash (data are 32-bit aligned).
  * @note   After writing data buffer, the flash content is checked.
  * @param  FlashAddress: start address for writing data buffer
  * @param  Data: pointer on data buffer
  * @param  DataLength: length of data buffer (unit is 32-bit word)   
  * @retval 0: Data successfully written to Flash memory
  *         1: Error occurred while writing data in Flash memory
  *         2: Written Data in flash memory is different from expected one
  */
uint32_t FLASH_If_Write(__IO uint32_t* FlashAddress, uint32_t* Data ,uint16_t DataLength)
{
  uint32_t i = 0;

  for (i = 0; (i < DataLength) && (*FlashAddress <= (USER_FLASH_END_ADDRESS-4)); i++)
  {
    /* Device voltage range supposed to be [2.7V to 3.6V], the operation will
       be done by word */ 
    if (HAL_FLASH_Program(FLASH_TYPEPROGRAM_WORD, *FlashAddress,  *(uint32_t*)(Data+i)) == HAL_OK)
    {
     /* Check the written value */
      if (*(uint32_t*)*FlashAddress != *(uint32_t*)(Data+i))
      {
        /* Flash content doesn't match SRAM content */
        return(2);
      }
      /* Increment FLASH destination address */
      *FlashAddress += 4;
    }
    else
    {
      /* Error occurred while writing data in Flash memory */
      return (1);
    }
  }

  return (0);
}

What is the metter of volatile argument?

And another part. I think volatile has no reason in it.

// Check if valid stack address (RAM address) then jump to user application
if( ((*(__IO uint32_t*)MAIN_PROGRAM_START_ADDRESS) & 0x2FFE0000 ) == 0x20000000 )
{
    /* Jump to user application */
    uint32_t jumpAddress = *(__IO uint32_t*) (MAIN_PROGRAM_START_ADDRESS + 4);
    /* Initialize user application's Stack Pointer */
    __set_MSP(*(__IO uint32_t*) MAIN_PROGRAM_START_ADDRESS);
    ((pFunction)jumpAddress)();
    /* do nothing */
    while(1);
}

ST uses __IO to hide volatile.

#define __IO volatile  // from main uC header, i.e. "stm32f4xx.h"