The Torah's foundational command — Vayikra 16:29 — that Israel must "afflict their souls" on the tenth of the seventh month establishes suffering on Yom Kippur as an eternal statute, and Rav Melamed (Peninei Halakhah, Days of Awe 9:1) explains that this inui is fulfilled primarily through abstaining from food and drink, to which four additional abstentions — washing, anointing, sandal-wearing, and marital relations — are added, all constituting forms of deprivation, and the Shulchan Arukh (Orach Chayim 611:1) codifies this same list as the prohibited acts of the day.
Several Rishonim converge on the explanation that fasting subdues the body so that the soul may rise toward truth and atonement: Sefer HaChinukh 313 reasons that food, drink, and sensory pleasure drive physical matter toward desire and sin while negating the wise soul's pursuit of truth, making abstention essential on the day of judgment, and רבינו בחיי, כד הקמח כפורים תענית similarly argues that fasting weakens the animal faculty so that the light of intellect can shine and one's service and prayer become acceptable.
The Abarbanel extends this further, teaching in Abarbanel on Torah, Vayikra 23:26 that through the weakening of material desire on the fast day the soul's longing for enduring things is strengthened, and that a person should intend during the fast that his spirit and soul are themselves an altar of atonement, with his own inner heat consuming his fat and blood in service of his Creator, while the Sforno (Vayikra 16:30) adds that abstention and affliction are needed because the priestly service alone achieves only a diminishment of sin, readying it to receive forgiveness — not full atonement on its own.
The Sefat Emet articulates the deeper theological root: Sefat Emet, Yom Kippur 5655 teaches that souls are in truth close to God, and it is sin alone that brings distance and separation — citing the verse that iniquities form a barrier between a person and God — so that the inui of Yom Kippur serves to dissolve that separation, and Rav Melamed (Peninei Halakhah, Days of Awe 6:6) elaborates that throughout the year the soul is covered by a material layer arising from bodily desires, and the fast is commanded precisely so that the soul may detach somewhat from the bonds of the body and its deepest aspirations be freed and revealed.
The Mishnah makes clear that suffering alone is insufficient, because Yom Kippur atones only together with repentance — as Mishnah Yoma 8:1–9 states explicitly that Yom Kippur does not atone for one who says he will sin and rely on the day to forgive him — and Mishneh Torah, Repentance 2:7–8 rules accordingly that Yom Kippur is the apex of forgiveness and pardon for Israel precisely because everyone is obligated to repent and confess on it.
The prophetic reading for Yom Kippur from Yeshayahu 58:3–7 challenges the notion that bodily affliction is sufficient in itself, asking whether starving the body and bowing like a bulrush is truly the fast God desires, and answering that the chosen fast is the release of the oppressed — a reminder that the inui of Yom Kippur is meant to catalyze genuine moral transformation, a theme reinforced by the narrative of Yonah 3:5–10, in which the people of Nineveh fast and cry out to God while each turning back from his evil way, and God relents.
The Maharal frames suffering as one of three paths to repentance: Maharal, Netiv HaTeshuvah 1–2 distinguishes those who return from love (the highest level), those who return from fear, and those who return on account of suffering, with the last representing the lowest rung — yet still a genuine and divinely-orchestrated return, since God, as it were, compels the person through affliction as a master brings a wayward servant back.
Rav Melamed synthesizes the purpose of suffering within the larger architecture of atonement: Rav Melamed (Peninei Halakhah, Days of Awe 6:1) explains that because the foundation of atonement is granted by God at His will, Yom Kippur illuminates Israel's inner goodness and cleanses the roots of their souls even without complete repentance — but because the sins themselves remain, suffering is necessary to neutralize their effects, and the more genuinely people repent, the more Yom Kippur's purification extends to the ramifications of sin, rendering further suffering unnecessary.
The Gemara records that Rav Hamnuna Zuta's Yom Kippur confession included the petition "erase [my sins] in Your compassion, but not by suffering" — Yoma 87 — which presupposes that suffering is one legitimate mechanism for the erasure of sin, while simultaneously expressing the hope that repentance and the day itself will make it superfluous, a prayer whose logic depends on the broader Talmudic teaching in Berakhot 5a that suffering prompts a person to examine his deeds and return to God.
רש"ר הירש, במעגלי שנה ב עמוד פח adds a dimension of existential humility: only when our hands are bound from labor and our bodies stripped of their pleasures are we properly equipped to stand at the gate on this day and petition for permission to continue to exist — the inui expressing the recognition that without God's constantly renewed mercy, our sins would long since have destroyed us.
Heichal Yitzchak (Heichal Yitzchak OC 61) holds that fasting on communal fast days is designed to awaken the heart to repentance and serves as a concrete reminder of the wrongdoings — both personal and ancestral — that brought about the troubles those days commemorate.