We light the altar candles only when we have communion.
I tell my acolyte class that there are a couple of reasons we follow this practice:
1. It's a visible reminder that we will have the special presence of Christ in communion.
2. The two candles visibly remind us of the unique nature of Jesus as fully God and fully man. Which is important because as a man he has a body with blood like ours that he gave as the once-for-all sacrifice for our sins, and as God he can be present in the sacrament as he says he is.
3. The two candles visually remind us of the bread and the wine by which we receive Jesus' body and blood as he promises in his own words of the new covenant.
Of course, candles and they are lit or not is among the many things that are adiaphora in worship, things that the Bible neither speaks for or against.
Hope this helps.
Jim Neubauer
------------------------------
Jim Neubauer
St. Johns Lutheran
------------------------------