Does there have to be an explainable purpose? It seems God didn’t state His purpose behind prohibiting Adam and Eve the eating from a certain tree.
It also seems Jesus didn’t tell us to fast before Communion…
Yes, it has been a Church discipline and that is fine. But if we are to inform our conscience it is ok to ask why rather than just do something without purpose.
The most important point regarding this question concerns why we ought to fast. St. Paul reminds us, “Continually we carry about in our bodies the dying of Jesus, so that in our bodies the life of Jesus may also be revealed” (II Cor 4:10). We, too, are charged to convert our whole lives — body and soul — to the Lord. This conversion process involves doing penance — including bodily mortification like fasting — for our sins and weaknesses, which in turn strengthens and heals us. Pope Paul VI exhorted the faithful in his apostolic constitution “Paenitemini” (1966), “Mortification aims at the ‘liberation’ of man, who often finds himself, because of concupiscence, almost chained by his own senses. Through ‘corporal fasting’ man regains strength, and the wound inflicted on the dignity of our nature by intemperance is cured by the medicine of a salutary abstinence.”
Moreover, the fast before receiving holy Communion creates a physical hunger and thirst for the Lord, which in turn augments the spiritual hunger and thirst we ought to have.
holyspiritinteractive.net/columns/williamsaunders/straightanswers/104.asp
Fasting can be tricky. We can easily start to feel self righteous and look down on others.
Luke 18:11-14
The Pharisee took up his position and spoke this prayer to himself, ‘O God, I thank you that I am not like the rest of humanity - greedy, dishonest, adulterous - or even like this tax collector. I fast twice a week, and I pay tithes on my whole income.’
But the tax collector stood off at a distance and would not even raise his eyes to heaven but beat his breast and prayed, ‘O God, be merciful to me a sinner.’
I tell you, the latter went home justified, not the former; for everyone who exalts himself will be humbled, and the one who humbles himself will be exalted."
Matthew 9:13
Go and learn the meaning of the words, ‘I desire mercy, not sacrifice.’ I did not come to call the righteous but sinners."