Why did God turn Moses' rod into a serpent as a sign?
The sign was given so the Israelites might believe that the God of their fathers—Abraham, Isaac, and Jacob—had appeared to Moses and that he was God's man. When the rod became a serpent and then a rod again, it authenticated Moses' commission before Israel.
Les explains that when Israel was now a nation and God was beginning to work with them, He sent Moses back to the people under slavery in Egypt. God said that if they did not believe Moses, he should throw the rod on the ground and it would become a serpent; when he picked it up, it would become his rod again. God also gave Moses the sign of the leprous hand, which became perfectly whole again. These signs were given to Moses to prove to Israel that he was God's man, so they might believe the God of Abraham, Isaac, and Jacob had appeared unto him. Les notes that Israel was used to such supernatural events throughout their history, unlike people today who would be overwhelmed by an angelic appearance.