If Jesus had stopped at verse 24, we still would have a powerful example of unmerited grace--a father forgiving his son and celebrating his return. When we sin, we can find hope that we will be forgiven. When someone sins against us, we can find an example of how to show forgiveness.
But, Jesus did not stop with the celebration.
Rather, he introduced the elder brother. Like the Pharisees and scribes who had been complaining that Jesus was associating with sinners, the elder brother had always been obedient. And, like them, he wasn't happy at all about the inclusion of someone who had not exhibited much obedience.
He's particularly upset by the extravagant celebration. "I've done everything you could have expected, and now, you are giving a dinner for him!"
The father reminded the elder son that he still was going to get everything that he had been expecting to get. Celebrating the return of the younger brother did not change the status of the elder brother. "But," the father insists, "You need to be happy about his return. He was lost to us and now has been found."