As gluttony can lead your soul to sin, fasting can help lead you away from sin
Fasting is contrary to that fullness of bread, which makes both body and soul more disposed to vice, and indisposed to religious duties, through drowsiness of head, heaviness of heart, dullness and deadness of spirit. Now these being removed, and the dominion of the flesh subdued by fasting, the body will be brought into subjection to the soul, and both body and soul to the will of God, more readily than otherwise they would be.
Fasting can aid your spiritual exercises
A day of fasting is a great assistance to the soul, for the better performing of holy duties, such as meditations, reading, and hearing the Word, prayer, examining, judging, and reforming a person's self; both because his spirits are better disposed, when he is fasting, to serious devotion; and the mind being so long taken wholly off from the thoughts, cares, and pleasures of this life, he may be more intent and earnest in seeking of God.
Fasting summed up
" Fasting is an open profession of guiltiness before God, and an expression of sorrow and humiliation; being a real acknowledgement of man's unworthiness, even of the common necessaries of this present life."
Both your body and soul must “fast”
But it is not enough that the body be chastened, if the soul be not also afflicted, Isa 53:5; because it is else but a mere bodily exercise which profits little; nay, it is but an hypocritical fast, abhorred and condemned by God, frustrating a chief end of the fast, which is that the soul may be afflicted.
Afflicting the soul works repentance; another chief end and companion of fasting; for godly sorrow causes repentance, never to be repented of, 2 Cor 7:10.
When the soul is afflicted and heavy laden with sin, a man will readily and earnestly seek after God, even as the sick do to the physician for health, and as a condemned man to the king for a pardon. In their affliction (saith God) they will seek me diligently, Hos 5:15. If this be true of the outward, then much more of inward affliction.
Spiritualbenefits you can attain in fasting
The afflicted soul is a fit object of God's mercy; to him doth God look that is poor and of a contrite spirit, Isa 66:2, that trembles at his word; yea, the bowels of his fatherly compassion are troubled for him, Jer 31: 20, who is troubled and ashamed for his sin.
Moreover, upon a day of humiliation (if a man deal sincerely) this affliction of his soul drives him quite out of himself to seek help of God in Christ, and makes him endeavour to bring his soul into such good frame that he may truly say he doth not regard iniquity in his heart, Ps 66:18, and that his unfeigned purpose is, and endeavour shall be, to keep a good conscience toward God and man always. Whence follows boldness and assurance, through Christ Jesus, that God will be found of him, John 15:7, and that in God's own time, and in the best manner, he shall have all his holy desires fulfilled.