Why use the Bitwise-Shift operator for values in a C enum definition?

This way you can add multiple flags together to create a "set" of flags and can then use & to find out whether any given flag is in such a set.

You couldn't do that if it simply used incrementing numbers.

Example:

int flags = kCGDisplayMovedFlag | kCGDisplaySetMainFlag; // 6
if(flags & kCGDisplayMovedFlag) {} // true
if(flags & kCGDisplaySetModeFlag) {} // not true

Maybe writing the values in hexadecimal (or binary) helps :-)

enum {
  kCGDisplayBeginConfigurationFlag  = (1 << 0), /* 0b0000000000000001 */
  kCGDisplayMovedFlag               = (1 << 1), /* 0b0000000000000010 */
  kCGDisplaySetMainFlag             = (1 << 2), /* 0b0000000000000100 */
  kCGDisplaySetModeFlag             = (1 << 3), /* 0b0000000000001000 */
  kCGDisplayAddFlag                 = (1 << 4), /* 0b0000000000010000 */
  kCGDisplayRemoveFlag              = (1 << 5), /* 0b0000000000100000 */
  kCGDisplayEnabledFlag             = (1 << 8), /* 0b0000000100000000 */
  kCGDisplayDisabledFlag            = (1 << 9), /* 0b0000001000000000 */
  kCGDisplayMirrorFlag              = (1 << 10),/* 0b0000010000000000 */
  kCGDisplayUnMirrorFlag            = (1 << 11),/* 0b0000100000000000 */
  kCGDisplayDesktopShapeChangedFlag = (1 << 12) /* 0b0001000000000000 */
};

Now you can add them (or "or" them) and get different values

kCGDisplayAddFlag | kCGDisplayDisabledFlag /* 0b0000001000010000 */