The good news of the gospel always applies and affects Christians, even ones who've been Christians for a very long time.
I heard someone say once that he's heard the gospel story, so he'd rather not hear it again. He knows it. So why mention it again?
A major reason for needing to hear it again and again is that we all have a human nature. All of us have a nature that leans toward distraction and chasing after trivial things at times. So we need to be reminded of God's part in our lives then and now because of Christ's work for us.
Often when we hear how we were not worthy of fellowship with God but we are through Jesus's work and faith, we feel compelled to praise and worship and do good works.
We are also learning over time. When we read about certain accounts of Jesus in the gospels, we might see some detail that really shows us more the mind or love of Christ. Or how it relates to certain situations in our daily lives.
Not only in preaching or worship songs can we be reminded of the gospel, but it's important to include this in our devotional times too. Some devotions may only speak of how we need to do better--but really we need to be sure at times that we think about Christ being the reason we don't have to constantly think we have to do better. We can rest in him. We can know that everything will end well in a sense. And that often helps give us the peace and courage to do what we're commanded by God. There are times when we do need to think about doing better, but that shouldn't fill up the entirety of our devotional time. There is a place for worship too.