This happens in people's lives - for instance, if a fire destroys a home or some property. Also, scientists tell us about the changes that happen in physical matter because of fire. Fire changes things. In several ways, this biblical story is about change. As we imagined, perhaps Moses approached the fire to change from being cold to being warm.
Fire puts off heat.
It changes temperature. But in this particular story, the most significant change happened in the person of Moses himself. He was changed from being a shepherd to being the leader of God's people. He would not deal with flocks of sheep anymore after this encounter with fire. Rather, he would deal with God's flock.