There is a mitzva to light a candle to honor Shabbat.
There are three reasons for this: 1) to honor Shabbat, as a banquet without light is of no significance; 2) for oneg Shabbat, because one who cannot see his food does not enjoy it; 3) to bring shalom bayit (peace in the home), because one who cannot see his furniture and belongings trips over them and gets angry and irritable.
It is so important to have light at the Shabbat table that the Sages stated that one who does not have enough money to buy a candle should go door to door begging for charity in order to buy it.
One who has only a bit of money should first buy bread so he does not fast on Shabbat. After that, if he still has money left, he should buy a candle, and only then, if he can, should he buy wine for kiddush. This is because one may make kiddush over bread if necessary, and the light allows him to honor and enjoy Shabbat. It is more important to light a candle to honor Shabbat than to beautify kiddush with wine
The Shabbat candles give profound expression to the essence of Shabbat. One mired in darkness cannot find what he is looking for; he stumbles over his furniture; his whole home seems chaotic to him.
When we look at the world superficially, it seems full of strife and war, hopelessly divided and conflicted. Each side thinks that only when it succeeds in getting rid of the opposition will it be able to rest, and thus the conflicts endlessly continue. But if one thinks a little more deeply and examines divine providence, the darkness disappears and the divine light is revealed. He realizes that the opposing sides actually complement one another, and there is a hand directing and leading the world toward perfection. Out of all the troubles and afflictions, redemption and comfort will emerge The Shabbat candles, which bring peace to the home by adding light, thus allude to the repair of the world that comes about by increasing the light of Torah and faith. This is the goal of Shabbat – to add the light of faith and Torah to world.