Web API return type difference : HttpResponseMessage and Customtype/string?

when we are using HttpResponseMessage as return type of a action/method.

we can able specify the cache duration. cache duration, maintain cache or not...under control of developer it.

   public HttpResponseMessage Get()
   {
    HttpResponseMessage response = Request.CreateResponse(HttpStatusCode.OK, "value");
    response.Content = new StringContent("hello", Encoding.Unicode);
    response.Headers.CacheControl = new CacheControlHeaderValue()
    {
        MaxAge = TimeSpan.FromMinutes(20)
    };
    return response;
} 

Response Header:

HTTP/1.1 200 OK Cache-Control: max-age=1200 Content-Length: 10 Content-Type: text/plain; charset=utf-16 Server: Microsoft-IIS/8.0

when we using customtype/string as return type?,

Why HttpResponseMessage and why not directly return type is : string/Employee/IEnumerable ..ect ?