The Stick of Musa turned into a Snake
This was a proof from Allah for Musa and a great miracle. This was something that broke through the boundaries of what is considered normal, thus, it was a brilliant evidence that none but Allah could do. It was also a proof that no one could come with the likes of this (from mankind) except a Prophet who was sent (by Allah).
Concerning Allah's statement,
وَمَا تِلْكَ بِيَمِينِكَ يَا مُوسَى
And what is that in your right hand, O Musa!
Some of the scholars of Tafsir have said,
"He (Allah) only said this to him in order to draw his attention to it."
It has also been said,
"He only said this to him in order to affirm for him what was in his hand. In other words, that which is in your right hand is a stick that you are familiar with. You will see what We are about to do to it now."
وَمَا تِلْكَ بِيَمِينِكَ يَا مُوسَى
And what is that in your right hand, O Musa,
This is an interrogative phrase for the purpose of affirmation