God did not "need" Moses to raise his arm to part the Red Sea. And He did not "need" the Israelites to march around Jericho so that He could bring the walls down. God did not "need" the boy David or a slingshot to slay Goliath. In each case God moved His own people to act on faith and then He made it so.
Therefore I say unto you, What things soever ye desire, when ye pray, believe that ye receive [them], and ye shall have [them]. And when ye stand praying, forgive, if ye have ought against any: that your Father also which is in heaven may forgive you your trespasses. But if ye do not forgive, neither will your Father which is in heaven forgive your trespasses. – Mark 11:24-26
It is neat how God uses us as his vessels so his grace can flow through.