John Calvin Commentary Exodus 4:17

"And thou shalt take in thy hand this rod, wherewith thou shalt do the signs." — Exodus 4:17 (ASV)

And you shall take this rod. There is no doubt that God chose this shepherd’s rod to be the instrument of His power, in order to confound the pride of Pharaoh all the more. For what but shame and reproach could it bring to Moses, that he should carry with him the crook with which he had previously guided his sheep in their folds and hovels? This symbol, then, of a rustic and contemptible occupation, was opposed to the scepter of Pharaoh, not without humiliation.

In this respect, therefore, the obedience of Moses is worthy of praise, because he is not ashamed of a lowly and humble appearance, but willingly carries his rod, and thus makes himself as nothing, and glorifies God. So God is usually accustomed to hide His treasures in earthen vessels, and to choose the weak things of the world to confound the things that are mighty.

But from Moses being commanded to work the miracles with the rod, we gather that outward signs are often used by God when He works by His own hand; not to detract at all from His power, or to obscure His praise, but to make it manifest that the whole world is subject to Him, and that He freely applies to whatever use He pleases, things which are otherwise of no account.