In the desert (at the present time of our text), the signs are to help Moses obey in faith. In Egypt, the signs are to help the Israelites believe that God has appeared to Moses, but also to show God’s power to Pharaoh (see Exodus 3:20 and Exodus 4:21). The first and second signs are done with objects in the desert (staff, hand), the third can only be performed in Egypt.
A prophet can do wonders, but the important part is the message (Deuteronomy 13:1–5). In 1 Corinthians 1:22–24 we read that Jews demand signs and Greeks seek wisdom, but we preach Christ crucified, a stumbling block to Jews and folly to Gentiles, but to those who are called, both Jews and Greeks, Christ the power of God and the wisdom of God.
1 Then Moses answered, “But behold, they will not believe me or listen to my voice, for they will say, ‘The LORD did not appear to you.’”