We know Seraphim and Cherubim are in the closest in proximity to the Father, in the inner court holding up His throne, but whey does the Bible use the word "guard" in their "job description"?
Hebrews Chapter 9 provides the key
Then verily the first covenant had also ordinances of divine service, and a worldly sanctuary.
2 For there was a tabernacle made; the first, wherein was the candlestick, and the table, and the shewbread; which is called the sanctuary.
3 And after the second veil, the tabernacle which is called the Holiest of all;
4 Which had the golden censer, and the ark of the covenant overlaid round about with gold, wherein was the golden pot that had manna, and Aaron's rod that budded, and the tables of the covenant;
5 And over it the cherubims of glory shadowing the mercyseat;
of which we cannot now speak particularly.
The role of the 2 cherubims is to guard the entrance to God's presence. Sinful men, possessing Adam's sin nature, could not enter into God's presence without being destroyed by the 2 of them.
But once Jesus paid the final price for our justification, the 2 cherubims no longer needed to guard God from us. We can now confidently walk into the presence of God, once we have the blood of Christ in our hearts. That is why Hebrews 9:5 says
which we cannot now speak particularly.