Why use !! when converting int to bool?

Question

What can be a reason for converting an integer to a boolean in this way?

bool booleanValue = !!integerValue;

instead of just

Answer 1

The answer of user143506 is correct but for a possible performance issue I compared the possibilies in asm:

return x;, return x != 0;, return !!x; and even return boolean_cast(x) results in this perfect set of asm instructions:

test    edi/ecx, edi/ecx
setne   al
ret

This was tested for GCC 7.1 and MSVC 19 2017. (Only the boolean_converter in MSVC 19 2017 results in a bigger amount of asm-code but this is caused by templatization and structures and can be neglected by a performance point of view, because the same lines as noted above may just duplicated for different functions with the same runtime.)

This means: There is no performance difference.

PS: This boolean_cast was used:

#define BOOL int
// primary template
template< class TargetT, class SourceT >
struct boolean_converter;

// full specialization
template< >
struct boolean_converter
{
  static bool convert(BOOL b)
  {
    return b ? true : false;
  }
};

// Type your code here, or load an example.
template< class TargetT, class SourceT >
TargetT boolean_cast(SourceT b)
{
  typedef boolean_converter converter_t;
  return converter_t::convert(b);
}

bool is_non_zero(int x) {
   return boolean_cast< bool >(x);
}