I can only speak for Catholics. Some people who convert my choose to be re-baptized, but we believe Christian baptisms are acceptable.

Yes, it is a big deal. Read below but it is a celebration of welcoming the person officially into the Christian (Catholic-Christian in my case) community. Details follow, but for more go to americancatholic dot org.

Information from americancatholic dot org

Catholic view:
The ritual of Baptism does not bring God's love into being as if that love did not exist before the ceremony. Baptism is the Church's way of celebrating and enacting the embrace of God who first loved us from the moment of our conception. Baptism celebrates a family's and a community's experience of that love in the baptized.

There are other life experiences-birth, death, washing, growing and so forth-that are celebrated in Baptism. The water represents life, death, cleansing and growth, and it recalls the flood waters of Noah's day and the saving waters of the Red Sea parted by Moses. The candle symbolizes our status as an "easter people" and signifies the way that the Church "passes the torch" of Christian commitment to those being baptized. The white garment represents the Church's belief that Baptism sets us free from Original Sin.

Baptism happens not only to the individual, but also to Christ's body, the Church. That's why the rite insists that we celebrate Baptism in the Christian assembly, with the community present and actively participating. It is the community, after all, who is welcoming the new members, journeying with them, providing models for them, supporting and nourishing them. Baptism begins with God's love and care revealed to us through Christ. It continues with us, the Church, living and enacting God's love and care through Christ to the world. That's a serious commitment.

Baptism’s Biblical Roots
(by Carolyn Thomas, S.C.N.)

The practice of baptism was a common rite of initiation in many religious expressions in the ancient Mediterranean world. From the time of Jesus, Christianity also expressed through water baptism freedom from sin, union with Jesus Christ and all other baptized persons, our participation in the salvific death and resurrection of Jesus and our new life in the Spirit.

St. Paul, the first great theologian of baptism, expressed its meaning in terms of a break with the old and beginning of new life in Christ. He understood well the reality of the relationship that baptism establishes between us and God and his Son Jesus. The New Testament provides the basis and focal point for the Church’s understanding of baptism.

….Out of God’s revealed word, the Church has developed a theology of baptism that takes into account the lived experience of the Church throughout its history, its liturgical life and its theological developments.

The notion of baptism as a sacrament dates back to the early centuries of Christianity. The word "sacrament" is borrowed from the Latin, sacramentum, which in Roman times referred to an initiation rite in which soldiers promised their fidelity to their commander. In teaching Gentiles, the Church used the word sacramentum to explain the rite of Christian initiation in which the initiates would commit themselves to the service of God. When Christianity supplanted polytheism in the empire, the Roman sense was dropped, and the word was expanded to any symbol that represented one’s relationship to God.

By the fifth century, St. Augustine referred to a sacramentum as anything that was "a sign of a sacred reality." By the twelfth century, the word was restricted to the seven rituals of the Church which Catholics refer to as the seven sacraments…..

In Eastern Orthodox Christianity this is an act that is supposed to convey grace from what I remember