What was the main theological objective for Ezekiel’s depiction of the celestial chariot in Ezekiel 1?
This is a great question. As you probably know the prophets are very difficult to interpret. They write in very allusive language and their words are very poetic. With this stated, it seems that the theological objective of Ezekiel's description of the heavenly chariot in Ezekiel 1 is twofold.
First, it shows who really is in control. Within this context there are many powerful nations. Warfare is constant and God's people are small players. So, by showing God in a divine chariot, there is a sense that Israel's God is a divine warrior and greater than the powerful nations around them. In other words, they do not have to fear. Who can stand against a heavenly chariot?
Second and perhaps more importantly, God is calling Ezekiel to prophesy against this people. Ezekiel is understandably worried and has many fears. This is why God speaks these words to him in the very next chapter:
"He said: 'Son of man, I am sending you to the Israelites, to a rebellious nation that has rebelled against me; they and their ancestors have been in revolt against me to this very day.'"
In light of this, this heavenly vision acts as security that the calling of Ezekiel as a prophet is rooted in the power of a divine king. So, Ezekiel does not have to be afraid.