Asking why we need to be filled by Him is an important question for Christians to consider. One reason is that He empowers us to be a witness of Jesus, as stated in Acts 1:8, where Jesus tells the disciples that they will receive power when the Holy Spirit comes upon them. This power gives us strength, ability, and enables us to perform miracles, all for the purpose of going into all the world to be a witness of Jesus Christ.
Another reason is that He anoints us to do our purpose, as stated in 2 Corinthians 1:21-22. God makes us stand firm in Christ, anoints us, sets His seal of ownership on us, and puts His Spirit in our hearts as a deposit, guaranteeing what is to come. This anointing gives us the ability to fulfill the purpose that God has for us.
Being filled by Him empowers us to be a witness of Jesus and anoints us to do our purpose. It is essential for Christians to seek the Holy Spirit's filling to live a fulfilled life and fulfill their God-given purpose.