Why PestJSON require POST and PUT request and response WITH JSON data array ?

Some servers requires POST, PUT via HTTP POST and response like JSON for fix it i must add to rest server this lines:

if( $_SERVER['REQUEST_METHOD'] == 'POST' )
{
    if( $_SERVER['CONTENT_TYPE'] == 'application/json' )
    {
        // register POST if JSON content_type available
        $_POST = json_decode(file_get_contents('php://input'), true);
    }
}

BUT WHY ?
