Because those peole who offered sacrifice were also sinners before the Lord that’s why were repeated sacrifice for atoning.
Repeated animal sacrifices were necessary because people are sinners. Each year, to rid oneself of sin, they had to offer an animal to sacrifice to God.
Repeated sacrifices were important because God establish the spilling of blood as away to forgive sin. Jesus was not crucified yet, so they had to continue their sacrifice until Jesus arrival
All the sacrifices to remove sin were temporary and humans will continue to sin so it had to be repeated each year
Everything in the Old Testament points to the Messiah, Jesus Christ. All the sacrifices to remove sin were temporary, inadequate, to once and for all atone for sin and the resulting separation from God. Jesus then came as the ultimate fulfillment of those sacrifices, when He Himself, Who knew no sin became the PERFECT sacrifice that satisfied the justice of God and provided for the reconciliation of man to God.
In the old testament this practice was continually done because the new covenant had not been established yet. The relationship of the world to Jesus Christ hadn’t been established yet.
Humans, even saved humans, will continue to sin. The Bible says without the shedding of blood there can be no remission of sins. Each year a perfect spotless lamb would need to be sacrificed to make atonement for sin. When Jesus came his as both man and God his sacrifice was final. As a man he could stand in our place, but as God he could be the perfect, spotless, sacrifice needed once for all. He paid for all of our sin on the cross so we no longer need to continually offer a yearly sacrifice.