Because the Church is being prepared as the bride of Christ. We are a body now with relationships with each other, gifts to share and a mutual commitment to our head. But we shall be a bride then, with the wholeness of intimate relationship with our Lord. That is why He is committed to growing His body , because He is preparing us for the marriage feast of the Lamb. And that does not happen on our own.
Growth is linked to being part of the body.
The whole body belongs together, and Christians grow when they are part of the body. And so does the rest of the body. 1 Cor 12 says that God has arranged each part of the body just as He wants it. The church displays God s purposes to all the principalities and powers in the Heavenly realms. The church not only proclaims God wisdom by what it says, but by what it is.