For the same reason as good manners anywhere. When you are a guest at comeone's dinner table, good etiquette is the rule. So it is when we come to the Lord's Table, or visit His HouseThese devotional acts are bodily expressions of what we believe, symbols of our Holy Faith. To perform them is to worship God in an orderly and fitting manner - not with our lips only, but with the whole man.
"Stand for praise, sit for instruction, and kneel for prayer."
When the Celebrant enters the Sanctuary.
Gospel
Creed
Gloria in Excelsis
Hymns(Morning and Evening Prayer) When the Minister enters.
Hymns
All Gloria Patris
Venite
Psalms
Canticles
Creed
Epistle
Sermon(Morning and Evening Prayer) Psalms
Lessons
Sermon
Throughout, except when standing or sitting.
At the beginning and end of private prayers
At the words: "In the Name of the Father, and of the Son, and of the Holy Ghost. Amen"
Grace at meals.(Holy Communion) At the announcement of the Holy Gospel (thrice with the thumb on forehead, lips, and breast)
Creed: "...and the Life of the world to come"
Absolution: "...pardon and deliver you. etc."
Benedictus: "...Blessed is He that cometh, etc."
Prayer of consecration: "...Blessed is He that cometh, etc."
Prayer of Consecration: "...this is my Body...this is my Blood"
Lord's Prayer: "...deliver us from evil"
When the Celebrant turns to you with the Blessed Sacrament and says, "Behold the Lamb of God, etc."
Gloria in Excelsis: "...in the Glory of God the Father. Amen."
Blessing: "...the Blessing of God Almighty, the Father, the Son, etc."(Morning and Evening Prayer) Absolution: "...He pardoneth and absolveth, etc."
Creed: "...And the Life everlasting"
Lord's Prayer: "...but deliver us from evil"
Grace: "The grace of our Lord Jesus Christ..."
Upon entering and leaving your pew, and in passing the midst of the Altar - IF - the Blessed Sacrament is Reserved, and/or after the Consecration has taken place at Holy Communion. Otherwise a bow to the Altar suffices. A white or red light is your sign of our Lord's Presence in the Reserved Blessed Sacrament.(Holy Communion) Creed: "...and was incarnate by the Holy Ghost, etc."
When you approach and leave the Communion Rail.
As the Cross is carried in procession passes you.
For all prayers.
Whenever the Sacred Name of Jesus is mentioned.(Holy Communion) Creed: "...God", "...Jesus", "...worshipped and glorified."
Sanctus: (Deep Bow) "Holy, Holy, Holy, etc."
Prayer of Consecration: (Deep Bow) "For in the night in which he was betrayed, etc."
Gloria in Excelsis: "...God", "...we worship thee.", "...receive our prayer."(Morning and Evening Prayer) Gloria Patri: "Glory be to the Father, etc."
Creed: "...God", "...Jesus"