It speaks also of its human undesirability by men who would think to find Him in more appropriate and convenient places.
Fire is, after all, a fearful phenomenon.
Rabbinical commentary has it that anyone who is willing has no assurance that a turning back to past and comfortable categories of what one knew or understood before might be possible. It is supreme risk, but also supreme portent. That Moses turned, released God's call and initiated Israel's deliverance.