In the Bible, anointing someone with oil is performed in religious ceremonies and used as a special blessing, such as
1. For appointing kings (1 Samuel 16:3), prophets, and priests.
Jesus (the only Person who is king, prophet and priest) and is called the Anointed One. In all four Gospels we see a woman anointing Jesus.
2. Marriage (Ruth 3:3) and cleansing.
The Lord God requires Ruth to anoint herself before she marries Boaz.
3. Fasting (Matthew 6:17) for a holy purpose.
4. Healing the sick (James 5:14),
5. As a sign of hospitality to honored guests (Psalm 23:5).

We as Christians are in some sense anointed. We receive through Jesus an anointing from the Holy One (1 John 2:20). We partake in His anointing (2 Corinthians 1:21-22).

After so much Biblical evidence we feel free and called to bless the young couple by the physical sign of anointing, before their marriage is officiated in Christ the Lord.