Saint Augustine believed that creation was instantaneous because he had a faulty interpretation of Genesis 2:4, a mediocre interpretation of a faulty translation of an apocryphal text, and a belief that instantaneous creation is the best possible expression of omnipotence. However, God had other purposes in creation than just showing his power.
God created the world in six days and rested on the seventh day as a historical precedent for the Israelite work week. The King James version of Exodus 20:9–11 says “9Six days shalt thou labour, and do all thy work: 10But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates: 11For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.”