For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
M May
I love the thoroughness of the answer! Very educational! Thank you.
My answer is much more simplistic. God knew that Moses was going to resist the call with the excuses he later gives about being inadequate and throwing up road blocks to God's requests....can you imagine resisting God's instructions? But the simple answer is that God needed to get Moses' attention. He knew Moses needed emphatic proof of God's existence, holiness and power for the mighty call He was giving. Therefore, the need for such a dramatic presentation and visit from the Most Holy God as a theophany. Not sure I've added anything but that is what is revealed to me.