Short answer: to help Catholics understand the magnificent and sublime honor it is to receive Him, Body, Blood, Soul and Divinity in the Holy Sacrament of the Eucharist.
It just doesn't seem appropriate to munch on a Big Mac, and then, minutes later, become One Flesh with Christ, in the most profound and intimate union with the Godhead this side of heaven.
Longer answer: we have a foreshadowing of the Communion Fast in the Old Testament in the rituals performed by the High Priest of Israel. Once a year, on Yom Kippur, the high priest was permitted to enter the Holy of Holies, which was the inner sanctuary of the Temple, and approach the Shekinah (the glory of the Divine Presence), which dwelt in the Tabernacle (Ark).
But first he must scrupulously prepare himself for the awesome (yet terrifying) privilege of approaching the Holy Presence.
Seven days before Yom Kippur the High Priest would separate from his own household and take up residence inside the Temple, in order to perform a meticulous and rigorous ritual of preparation and purification. He fasted, purifying his mind, and performed numerous ablutions (ritual bathings) in order to prepare and sanctify his body. He confessed his sins. He shaved his entire body. He removed his old vestments and donned pristine white garments....all in preparation for entering the Tabernacle and being in the glory of the Divine Presence.
It thus seems appropriate that we, who now have the awesome (yet also terrifying) privilege of approaching the Holy Presence, actually becoming One Flesh with Him, should do even a little bit of what the High Priest was commanded to do. We, too, should be pure--with no mortal sin on our souls--and mentally prepared to become One with the Son.
Fasting for one hour before Communion seems to be but a small expression of the preparation we should do before approaching the Tabernacle of the Lord.
In fact, our brothers and sisters in the Eastern Orthodox Churches must fast for the entire morning prior to receiving Him, so the one hour fast for us Catholics seems minute in comparison.
"So that the Body and Blood of our Lord may be the first thing to pass our lips on the day of communion, we abstain from all food and drink from the time that we retire (or midnight, whichever comes first) the night before. Married couples should abstain from sexual relations the night before communion.When communion is in the evening, as with Presanctified Liturgies during Lent, this fast should if possible be extended throughout the day until after communion. For those who cannot keep this discipline, a total fast beginning at noon is sometimes prescribed." --"The Fasting Rule of the Orthodox Church"
The Catholic Code of Canon Law, Canon 919 states that “one who is to receive the Most Holy Eucharist is to abstain from any food or drink*, with the exception only of water and medicine, for at least the period of one hour** before Holy Communion.”
(*This includes gum
**The one hour fast is measured from the time a person would receive Holy Communion at Mass rather than the actual time the celebration of Mass begins; and for those who are sick, the fast is shortened to 15 minutes if possible)
While not required, some other ways to prepare for the privilege of receiving the Eucharist would be to engage in these pious practices:
-turn off the radio on the way to Mass as a way of consciously transitioning from the secular to the sacred
-arrive early in order to mentally and spiritually "fast" from the distractions of the day
-read the Sunday readings in advance
It is true that in the past the Catholic Church mandated that Catholics fast from midnight until reception of Holy Communion, and this discipline was changed after Vatican II to fasting for only an hour before Communion.
This mandate is a discipline, which is a pastoral practice of the Church, and not a doctrine,which is a teaching of the Church; and the Church is empowered to change its disciplines or practices. That is what is meant by the "power to bind and loose" that Jesus gave to the Church in Matthew 18:18
"whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven."
Thus, a change in discipline is not to be interpreted as saying, "Oh, so the Catholic Church has changed its teaching on fasting, right?" The teaching remains the same: Catholics must fast before receiving the Eucharist.
Some other examples of disciplines in the Church which have changed via the power to "bind and loose" are:
-abstaining from meat on Fridays
-a celibate priesthood
-the ban on cremation
So, while mandates for different pastoral practices of the faithful can indeed change, the doctrines of the Church,"given once for all to the saints" (Jude 1:3), have been the same for 2000 years and can never change.