Like many of our worship items, the candles on the altar as well as those sometimes placed around the lectern, began for practical reasons. Early Christians often met in secret, including in caves, cellars, & tunnels to avoid persecution. Those places were of course usually dark so candles were needed to see each other, & later to read.
These candles today are largely symbolic, reminding us of the presence of the Holy Spirit. While we now advertise when & where we meet, the altar candles ought to remind us of our Christian siblings who still worship in secret.