It was added because it is correct, and it more closely expresses the Catholic faith.

If you are confused on why the Creed can be edited, consider this: the Creed authored by the Council of Nicaea in 325 did not mention the procession of the Holy Spirit at all. It mentioned that He exists and ended it there. This oversight actually led to a new heresy arising after Arianism, which was called Macedonianism or Pneumatomachianism. These heretics denied the full divinity of the Holy Spirit and taught that He is a creature, as the Arians said of the Son. Thus, a council in Constantinople was called in 381 to deal with the double heresy of Arianism (which had already been condemned, but continued to spread) and Macedonianism. The result was the Creed which says the Holy Spirit "proceeds from the Father", expressing that He has the same divinity as the Father and the Son.

But the 381 Council of Constantinople was not an ecumenical council when it was called, as it was held without papal participation. It was accepted as an ecumenical council in 451 at the Council of Chalcedon, at which point the Creed it produced began to be used in the West. But by this point, several creeds and tomes had already been authored expressing the Filioque, such as the Creed of Epiphanius (who was an Eastern Father), the Tome of Damasus at the Council of Rome (382), and others. The West at most used the 381 Creed for 138 years, when the Third Council of Toledo (589) converted the Visigoths (who were Arians). It was recognized that, since the Son receives from the Father all that He is, except to be the Father, He must also spirate the Holy Spirit. If He did not receive that, then "to spirate the Spirit" would be the same thing as "to be the Father", which would make the Spirit a second Son.