What are the "entrance antiphons" and "communion antiphons" and why do we sing them at Mass?

The entrance antiphon and communion antiphons belong to what are called the Propers of the Mass. The Propers of the Mass are certain Scriptural texts, besides the readings, that are prescribed by the Church for the entrance and communion rites based upon the liturgical season and particular celebration of the day. These antiphon scriptural texts were started in the 6th century by Pope St. Gregory the Great and were re-issued by the Second Vatican Council to help us connect the three-year cycle of Mass readings with the sung entrance chant and the communion chant. Listening and singing along with these scriptural chants helps you to delve more deeply into the meaning of the Eucharistic liturgy and prepare you for hearing and responding to God’s spoken Word which you are about to hear in the Mass readings.
 
The Church allows both the entrance antiphons and communion antiphons to be sung by the cantor or choir with the congregation responding, the choir and cantor in alteration, or the choir alone. While the priest is receiving the Eucharist, the communion chant is begun, its purpose being to express the spiritual union of the communicants by means of the unity of their voices, to show gladness of heart and to bring out more clearly the “communitarian” character of the procession to receive the Eucharist. The Roman Missal goes on to say that the music at Communion may be extended appropriately to include a congregational hymn, singing by the choir or cantor, and/or instrumental music.