Why was Jesus anointed if he was God?

Jesus had two natures - both human and deity. Jesus as a man that was approved by God and so it was done publicly by both man and God.
Matt 3:16-17: “When He had been baptized, Jesus came up immediately from the water; and behold, the heavens were opened to Him, and He saw the Spirit of God descending like a dove and alighting upon Him. And suddenly a voice came from heaven, saying, "This is My beloved Son, in whom I am well pleased."

The Levitical Law required that all priests to be consecrated when they began their ministry at about 30 years of age (Num. 4:3). This encompassed washing and then anointing (Exod. 29:4-7; Lev. 8:6-30). Jesus had both done to him at the same time. One by the appointed man, John who was the forerunner and the other by the Father who is in heaven. John washed him and the Father anointed him by sending the Holy Spirit upon him.

This alighting was the Spirit resting on him unlike any other man.
Jn. 1:32: And John bore witness, saying, “I saw the Spirit descending from heaven like a dove, and He remained upon Him. I did not know Him, but He who sent me to baptize with water said to me, 'Upon whom you see the Spirit descending, and remaining on Him, this is He who baptizes with the Holy Spirit.' “And I have seen and testified that this is the Son of God.”
Jesus’ baptism was inaugurate him into ministry. Identified to Israel and declared to be the Son of God by identifying Him as the Son with a audible voice spoken by The Father from heaven

Luke 4:18-19 "The Spirit of the Lord is upon Me, because He has anointed Me to preach the gospel to the poor; he has sent Me to heal the brokenhearted, to proclaim liberty to the captives and recovery of sight to the blind, to set at liberty those who are oppressed; To proclaim the acceptable year of the Lord."
The Spirit would give his humanity courage, supernatural abilities when necessary, as well as the ability of an evangelist to reach the hardened people.

Acts 10:38 "how God anointed Jesus of Nazareth with the Holy Spirit and with power, who went about doing good and healing all who were oppressed by the devil, for God was with Him.”
He as a man was anointed with the Holy Spirit who did miracles in greater number and nature than any other prophet. As a human, He was in complete submission to God the Father and was anointed for His ministry work that would last approx. 3 years. Because he came as a man in submission the Father He as a man needed to be anointed like the prophets of old. The difference was that He had the fullness of the Spirit (Isa.11:2).