Why do we need Joseph Smith

When Christ visited the earth he taught and did many great things. he knew that he would leave the earth so he gave men power or priesthood the act in his name, this men are called the apostles they are a essential part of Christ church for its form they alone comes the Revelation to whole church, had the power to do ordnance like baptism,  and to call other officers in the church.

Regardless of the valiant efforts of Christ’s apostles and their faithful followers, the original church that Christ restored began to fade away. Members faced severe persecution and all but one of the apostles were martyred. This is a period called the Great Apostasy, when there was a “falling away” (2 Thessalonians 2:1-3) from the gospel Christ organized. The apostolic authority to bestow priesthood keys and to receive revelation for the Church was lost along with many precious teachings. Errors about His teachings crept into the church resulting in conflicting opinions and lost truths. This period is what we call the Great Apostasy.

The main things lost in the apostasy

1. The correct doctrines and teachings,  galatians 1:6-9,2 timothy 4; 3-4
2. they changed the ordnance's and convents,  Isaiah 24: 5
3. the true priesthood organization of Christ church , Ephesian 4:11-14

Here is  a sweet video about the affect of the apostasy and a man found the truth

Without authority or divine direction, Christianity struggled to survive with conflicting opinions on even the most basic teachings of the gospel. Without priesthood authority or the full gospel, people had to rely on human wisdom to interpret the scriptures, principles and ordinances. Many false ideas were taught as truth, and much of what we know about the true character and nature of God the Father, His Son Jesus Christ, and the Holy Ghost was lost. Essential doctrines like faith, repentance, baptism, and the gift of the Holy Ghost became distorted and important doctrines were lost entirely.

So the power of God was lost to earth and could only come back by heavenly intervention. That way Joseph Smith is so important. He was the human that was lead by God to restored the true gospel and priesthood authority to the earth.