Initially, the Sabbath candle was lit for very prosaic reasons: to make light for those who eat the Sabbath evening meal, so that they would not spend the evening in utter darkness.
But, from the very start, the significance of candlelighting has gone far beyond that.
The Shulchan Aruch (Code of Jewish Law) rules:
“One ought to take care to make a nice candle… and some make two wicks, one for ‘Remember [the Shabbat day]’ (Exodus 20:8) and one for ‘Keep [the Shabbat day]’ (Deuteronomy 5:12)…”.
Indeed, it goes further:
“If one does not have enough to buy a candle for the Sabbath and wine for the kiddush of the day, the Sabbath candle takes precedence” — so much so that “even if one has nothing to eat, he is to beg for alms to buy oil and light the candle” (Shulchan Aruch,Orach Chayim 263).
The candlelight, then, has turned into the very symbol of the Sabbath itself, a sort of “light of the seven days” shining in a sanctified niche of time.
And just as the Sabbath enters with a light, so, too, we bid it farewell with a light: the Havdalah candle, a torch with which to escort the Sabbath Queen’s departure.
–Rabbi Adin Steinsaltz
From an essay titled “Shades of Light” by Rabbi Adin Steinsaltz