Ma'at was important for many reasons in Egyptian society. She plays a key role in most versions of the creation myth and is occasionally called "Mother of the Earth".
She was responsible for the balance of the universe and was usually said to be the cause of any fortune (good or otherwise)
Ma'at herself was sometimes thought of as a concept and not a physical being. If someone was fair and just in daily life, they could be called "ma'at". She was not just the Goddess of truth and justice, she IS truth and justice.
Though Ma'at was rarely worshipped in temples, it was important to try and please her on a regular basis. She was the fulcrum on which a spirit was weighed and judged in the afterlife and was therefore constantly present in an Egyptian's every day life, even the Pharaoh would consider himself a priest of Ma'at.