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This is an honest question by many Protestants.

Quite simply the doctrine was believed for hundreds of years yet had never been dogmatically defined. Basically, that means there is no more debate.

At the time of the declaration as dogma of the Assumption of Mary, there was much debate due to the influence of communism and its rejection of religion in general, Catholicism in particular. Mary had warned of the rise of communism and encouraged Catholics to pray for its fall. The Church declared that which the Church had held to be true for many years, was no longer up for debate, Mary was in heaven with her Son.

There are relatively few dogma that have been formally defined as absolutely binding upon a Catholic.

The Holy Trinity: A Catholic must believe in the Trinity of God the Father, God the Son, and God the Holy Spirit, Three Persons, One God

The Fall and Original Sin: A Catholic must believe that there was a great fall and in original sin.

The Incarnation, Redemption and the Real Presence: A Catholic must believe that Jesus, is the Incarnate Word of God and that He was born of a virgin, suffered and died on the cross and arose again. Our redemption is gained only by the Incarnation. A Catholic must believe that Jesus is real and truly present in body, blood, soul and divinity in the Eucharist.

The Seven Sacraments:Baptism, Confession, Communion, Confirmation, Marriage, Holy Orders and Extreme Unction.

The Church: A Catholic must believe that the Church was founded by Jesus, and that it is one, holy, catholic and apostolic.

Mary: A Catholic must believe that Mary was born free from original sin due to the grace of God and the nature of her Son. A Catholic must also believe that Mary was ever virgin and that she was assumed into heaven by her Son upon her death.

Everything else is just a filling out of these dogma and this is known as doctrine. For example, the nature of Jesus would fall under the dogma of the Holy Trinity. The Magisterium, church hierarchy(including the nature of each office) and infallibility of the pope would fall under the dogma of the Church which derives its authority from Scripture and Tradition under the perfect guidance of the Holy Spirit. And of course those doctrines relating to Mary would fall under the dogma of Mary.

Each of these was formally declared due to heresies that caused the faithful to lose faith.

This is but a rudimentary explanation. Further and far better edification, if one is truly interested, can be found in reading the Catechism, early Church Fathers and Doctors and other Catholic material which had the imprematur of a bishop showing it is free from error.