Why did Moses use signs and miracles before Pharaoh?
Moses used signs and miracles because, as Paul writes in 1 Corinthians 1:22, the Jews require a sign. Those miraculous proofs were given to authenticate Moses as a representative of the God of Israel, first to the children of Israel and later to convince Pharaoh.
Les explains that this is the beginning of what Paul claims in 1 Corinthians 1:22, where the Jews require, or demand, a sign. Beginning with Moses going to Pharaoh, or to the children of Israel first, and later to convince Pharaoh that he was representative of the God of Israel, Moses used signs and miracles as proof. Les notes that all through Israel's history the supernatural and miraculous happened over and over.