Why do we need a bishop?

Here is some information about bishops and their role in the Church and their diocese, as referenced in the Catechism of the Catholic Church (CCC):

A bishop is a member of the assembly of bishops, who stand in an unbroken line of successors of the original Twelve Apostles, with the Pope — successor of St. Peter, the rock upon whom Christ built His Church — at the head. (see CCC 880-882, 1555)

This body of bishops and its individual members many only exercise authority in the Church in unity with the Pope. (CCC 883)

This worldwide assembly of bishops is an expression of the variety and unity of the People of God, assembled under one head. (CCC 885)

As did the Twelve Apostles, bishops experience a particular outpouring of the Holy Spirit at their consecration as bishops, at which time they receive the fullness of the sacrament of holy orders. (See CCC 1555-1558)

With this grace of the Holy Spirit, a bishop participates the ministry of Christ as teacher, shepherd and priest. (CCC 1558)

A bishop is a lawful pastor of his diocese. (See CCC 1560) As such, he is a visible source and foundation of unity in his individual diocese. He exercises the pastoral office over the portion of the Church assigned to him, with the help of priests and deacons. (See CCC 996)

In the Latin Rite, the bishop retains the authority to ordain priests and administer the sacrament of confirmation. He consecrates the Sacred Chrism, the oil used in baptism and holy orders, for the whole diocese.