1) It shows us a just God. Reveals who we are and our need. God must be just. He chooses to be merciful at his great pleasure. There must be justice, mercy without justice is meaningless.
2) It explains the cross, the need for it. Reveals the real saving power.
3) Jesus on the cross is a stumbling block for many. They don't need repentance, man is basically good. The truth of Calvinism inflates the need for the for the gospel. THE GOSPEL IS NOT GOOD NEWS FOR ALL.
Nobody initially likes the truth of Calvinism. “THATS NOT FAIR, IS THE RESPONSE. But once you get to the full flavor of it, your relationship with God gets deeper. You understand who his is and who you are and what you deserve and what he offers.
You have just said the quiet part of Calvinism out loud:
The Gospel is not good news for all.
Woe to anyone who stands in the way of God’s grace to sinners.