Thus Moses warned Israel: “And guard your souls very much, for you did not see any form on the day that HaShem your God spoke to you at Horeb from out of the fire” (Deuteronomy 4:15). They were warned not to allow what they saw to cause them to err. They were to know that the entire vision was only prophetic, as they understood at the very time they saw it. Similarly, the rabbis said: “Since He was revealed to them at the sea as mighty (= Zeir Anpin), whereas at the Giving of the Torah He appeared in His attribute of kindness (= Arich Anpin), the verse states, ‘I am HaShem your God’, so as not to leave room to say there are two domains…” (Mechilta, Exodus 20:2).
This was so that they should not fall into error because of what they saw in the vision but that they should understand the underlying truth of what they saw in the proper way.