Leavened bread (that is, sourdough bread made by wild yeast) was eaten every day. Unleavened bread (made without yeast) became a sacred symbol. It reminded Israel of:
their suffering as slaves in Egypt (Deuteronomy 16:3),
the Passover (Exodus 12:17–20), and
their quick departure from that land (Exodus 12:34, 39).
Because of this meaning, unleavened bread became the bread used on the altar (Leviticus 6:17; 7:12). However, bread made with yeast could still be offered with a thanksgiving peace offering (Leviticus 7:13).