Israel, as God's servant, was to help bring the world to a knowledge of God. The Messiah, Jesus, would fulfill this task and show God himself to the world.
The nation of Israel is called the servant: Isaiah 41.8, 42.19, 43.10, 44.1,2,21, 45.4, 48.20.
The Messiah is called the Servant: 42.1-17, 49.3,5-7, 50.10, 52.13, 53.11.
The nation was given a mission to serve God, to be custodian to His Word, and to be a light to the Gentile nations. Because of sin and rebellion, they failed. God sent His Son, Christ as Messiah to fulfill His mission on earth.
Why is Israel the chosen ones? Because they are the first nation God revealed Himself to accomplish His eternal plan.
Israel's task is to be a witness (Isaiah 44.8) to all the nations telling the whole world who God is and what He has done.
Why choose this nation of all nations? Because they suffered the most, 400 years in slavery, or more accurately, were to suffer for 400 years in slavery, and because of Abraham a man of God (was willing to give up all that he loved in his son to forsake the flesh) received a son according to God's will through patience and of no expectation of his own with Sarah; his son was Isaac (a gift of God's grace), who beget a son Jacob (Esau gave up his own inheritance and let Jacob take it for Jacob so loved God), who beget a son Joseph that pleased the Lord (type of Christ who went into the gentile nation, Egypt, who was forsaken of his brothers and sisters, and yet embraced them yet again at some future time as will be the case in the coming new millennial kingdom saving a remnant of Israel).
And so began God's holy nation which literally stands apart from all other nations in God's will.
Joseph suffer at the hands of the Pharaoh. This repeated occurrence has been going on and on each century but it has ended in the Last Adam. There will not be a repeat after the Great Tribulation for the Last Adam has already come as we wait for His return to reign in the new millennial kingdom.