We are going to go back to Genesis 35:1. The context here is Jacob fleeing from Laban, returning to his homeland. He has just had the encounter with his brother Esau, and God took him through that.
When Jacob made that altar and began to make the offering, they were going to be in the presence of God. Clean yourself up, and change your clothes.
Genesis 35 and Exodus 19 are two examples of people dressing up in preparation for being in God's presence. In the first, Jacob voluntarily prepares, clearly following an already-established custom. In the second, God commands it. Dressing up to be in God's presence is a matter of respect, and God expects it. You can tell that especially from Exodus 19. It is a matter of respecting His holiness, and honoring the dignity of His office as Creator and Ruler.
