God was not obliged to redeem man. But having willed to do so, God the Father did not have to send Jesus to Earth for the salvation of mankind, as noted below from the 1912 Catholic Encyclopedia, volume 13, article: Salvation, page 407:
He [i.e., God the Father] might have done so [viz., redeemed man] by pardoning man’s sins without having recourse to the Incarnation of the Second Person of the Most Holy Trinity.
As the human race increased in number, the Father, in order to help souls reach Heaven, sent powerful leaders such as Moses, Abraham, Noah, and Isaac, and instructed man through the Old Testament and the Ten Commandments. Because of man’s weakness, concupiscence, and pride which promoted countless evils, God had to chastise man in the cities of Sodom and Gomorrah and to chastise the world with the great flood.
But with a great love for man, God did send His only-begotten Son to spend 33 years on earth. (The following is also taken from the 1912 Catholic Encyclopedia, volume 13, article: Salvation, page 407:
As Teacher, He established the reign of truth; as King, He supplied strength to His subjects; as Priest, He stood between Heaven and earth, reconciling sinful man with his angry God.
Over 33 years, He personally demonstrated what was necessary to save our souls.
Our Lord was sent to earth, born in poverty, and spent those 33 years teaching and personally living day-by-day, practicing many virtues: the Intellectual virtues of truth, wisdom, science, understanding; the Moral virtues of justice, fortitude, prudence, temperance, humility, meekness, modesty, patience; and the theological virtue of Charity and taught the theological virtues of Faith and Hope. This Life of virtue is how Our Lord lived His daily life demonstrating complete perfection of the above virtues. All to show us how to live day-to-day under all circumstances.
Our Lord did not only show us how to live, He also gave us an integral “gauge” to tell us how well we are doing toward saving our souls. It’s called a conscience. Wow! What a gift!
Because of Our Lord’s example, and using our conscience, man cannot excuse his poor behavior by claiming he cannot do better because he has no perfect example to use as a model for his conduct. Our Lord is that perfect example, demonstrating day-after-day, for 33 years, what is required of man for his salvation.
Consider what the world would be like without the above virtues being lived in practice, but instead with only pride, selfishness, aggression, greed, and physical strength dominating everywhere, much as the world was at the time of Our Lord’s Passion and Death. It would be a world where few would want to live and where no one would be happy or save his soul.
That is a great reason why we should thank God daily that He sent His only Son to earth to demonstrate just how we must live to achieve our goal of Heaven, after a happy life on earth. There can be no excuses (i.e., “no one showed us how to live”). He didn’t leave it only to verbal instruction. He showed us the way, the truth, and the life.
The following is from The Imitation of Christ, Book III, chapter 18.
The Voice of Christ:
My child, I came down from heaven for your salvation and took upon Myself your miseries, not out of necessity but out of love, that you might learn to be patient and bear the sufferings of this life without repining.
Oh, what great thanks I owe You, Who have shown me and all the faithful the good and right way to Your everlasting kingdom! … Had You not gone before and taught us, who would have cared to follow? …
Behold even we who have heard of Your many miracles and teachings are still lukewarm; what would happen if we did not have such light by which to follow You?