Orthodox
Why did Rome add the Filioque to the Creed?
The West added 'and the Son' to clarify, against Arian heresy, that the Spirit proceeds from the Father and the Son as from one principle, though the Church today acknowledges the addition was made without proper ecumenical process and works toward mutual understanding.
Catholic teaching
The Catholic Church teaches that the Father is the sole origin without origin of the Spirit's procession, but that the Son truly shares in that eternal procession, so 'Filioque' expresses a real theological truth even while the Church recognizes the Eastern formulation of 'through the Son' as a legitimate and complementary expression of the same mystery.
Biblical foundation
Jesus says the Spirit 'proceeds from the Father' (John 15:26) but also breathes the Spirit upon the apostles as his own gift (John 20:22) and says the Spirit 'will take what is mine and declare it to you' (John 16:14-15), texts both traditions read in their own theological framework.
Scripture references
Historical evidence
The phrase entered Western creeds locally in Spain in the sixth century to combat Arianism and was gradually adopted in the West, only becoming a major point of controversy with the East during and after the events of 1054.
The Church Fathers
“They [the Romans] have not made the Son the cause of the Spirit... but have manifested the procession through him.”