I always hear people speak of salvation in terms of what’s in it for them, but never in terms of what’s in it for God.
I guess it's just a human trait. We can know from Scripture that it was God who reached out to fallen mankind and provided a way to balance His mercy with His righteous holiness and justice. God does not wink at sin, in fact, God will not allow ANY sin into heaven - not so much as a lie. But he loved us so much that he made a way for us to be redeemed from our fallen state. God the Son (Jesus) took on human flesh, lived a sinless life and then sacrificially died FOR the sins of the world - past, present and future. He rose from the dead to prove Jesus was God incarnate and to show that his sacrifice was a once-for-all satisfaction for sin. He paid the penalty we deserved - which was death - and offers the gift of salvation to all those who would receive it through faith in Christ.
We know that God doesn't NEED anything, yet he created us knowing full well before hand what we would do and what it would cost to redeem us. Sadly, many people have no desire at all to know God or to accept his gift of everlasting life in heaven. That DOES grieve God. So, what's in it for God? That's a deep question but my thought is that through his love and mercy, as well as his leading and calling, he has souls he loves to spend eternity with and who will glorify and praise him forever.