Practical significance of Bitfield, union and Structure

/**
  \brief  Union type to access the Special-Purpose Program Status Registers (xPSR).
 */
typedef union
{
  struct
  {
    uint32_t ISR:9;                      /*!< bit:  0.. 8  Exception number */
    uint32_t _reserved0:7;               /*!< bit:  9..15  Reserved */
    uint32_t GE:4;                       /*!< bit: 16..19  Greater than or Equal flags */
    uint32_t _reserved1:4;               /*!< bit: 20..23  Reserved */
    uint32_t T:1;                        /*!< bit:     24  Thumb bit        (read 0) */
    uint32_t IT:2;                       /*!< bit: 25..26  saved IT state   (read 0) */
    uint32_t Q:1;                        /*!< bit:     27  Saturation condition flag */
    uint32_t V:1;                        /*!< bit:     28  Overflow condition code flag */
    uint32_t C:1;                        /*!< bit:     29  Carry condition code flag */
    uint32_t Z:1;                        /*!< bit:     30  Zero condition code flag */
    uint32_t N:1;                        /*!< bit:     31  Negative condition code flag */
  } b;                                   /*!< Structure used for bit  access */
  uint32_t w;                            /*!< Type      used for word access */
} xPSR_Type;

xPSR_Ttpe APSR;

uint32_t var1 = APSR.w;  // If we want to access as a word. Underneath Load Store Instruction
uint8_t var2 = APSR.b.IT;  //If we want to read from bit 25 and 26 only. Under neath Bitfiled instruction.

Hence union brings the freedom of access to a particular register.