Light symbolizes knowledge, and darkness ignorance. The Lord is the "Knowledge Principle" (Chaitanya) who is the source, the enlivener and the illuminator of all knowledge. Hence light is worshiped as the Lord Himself.
Knowledge removes ignorance just as light removes darkness. Also knowledge is a lasting inner wealth by which all outer achievements can be accomplished. Hence we light the lamp to bow down to knowledge as the greatest of all forms of wealth. Knowledge backs all our actions whether good or bad. We therefore keep a lamp lit during all auspicious occasion as a witness to our thoughts and actions.
Why not light a bulb or tube light? That too would remove darkness. But the traditional oil lamp has a further spiritual significance. The oil or ghee in the lamp symbolizes our "vaasnas" or negative tendencies and the wick, the ego. When lit by spiritual knowledge, the "vaasnas" get slowly exhausted and the ego too finally perishes. The flame of a lamp always burns upwards. Similarly, we should acquire such knowledge as to take us towards higher ideals.
A single lamp can light hundreds more just as a man of knowledge can give it to many more. The brilliance of the light does not diminish despite its repeated use to light many more lamps. So too knowledge does not lessen when shared with or imparted to others. On the contrary it increases in clarity and conviction on giving. It benefits both the receiver and the giver. A Saint has said
But the flame of a lovely lamp?
Which else shall adorn the mind
But the light of wisdom deep?
