Why does the Buddha compare the truth to a lamp?

The quote of Buddha and the lamp comes from Buddha's death statement to Anada. As he lay dieing he stated to his disciple -

"Be lamps unto yourselves. Be refuges unto yourselves. Take yourself no external refuge. Hold fast to the truth as a lamp. Hold fast to the truth as a refuge. Look not for a refuge in anyone besides yourselves. And those, Ananda, who either now or after I am dead, shall be a lamp unto themselves, shall betake themselves as no external refuge. But holding fast to the truth as their lamp, holding fast to the truth as their refuge, shall not look for refuge to anyone else besides themselves. It is they who shall reach to the very topmost height; But they must be anxious to learn."

In this he states that a lamp is parallel to the truth in many ways:

• Each person has his own lamp
• Each lamp comforts and aids the holder

It is interesting to note that the darkness that the lamp of truth dispels is the impalpable darkness of being unenlightened. Nothing really changes when the lamp is lit except the removal of absence.