Answer: These words “before me“, or before my face, in the First Commandment, teach us, that God, who sees all things, takes special notice of, and is much displeased with, the sin of having any other God: that so it may be an argument to dissuade from it, and to aggravate it as a most impudent provocation₁: as also to persuade us to do as in his sight, whatever we do in his service.₂

1. Ezekiel 8:5-8; Psalm 44:20-21
2. 1 Chronicles 28:9

For further study see:

1. WSC 48: What are we specially taught by these words, “before me” in the First Commandment?
2. Pastor J.J. Lim, Pilgrim Covenant Church, Singapore,  “The First Commandment”