Why are there often internal and external include guards?

[dneise]

Often one finds external include guards like:

#ifndef MARS_MInputStreamID
#include "MInputStreamID.h"
#endif

While the file being included already has an internal guard

------ contents of MInputStreamID.h -------
#ifndef MARS_MInputStreamID
#define MARS_MInputStreamID

/* Definition of class MInputStreamID */

#endif

Is this needed by the ROOT CInt, or is it just a historical left over?