A sinners’ prayer is basically a prayer of repentance, right? The term is mostly an evangelical thing, but prayer itself is ultimately conversation with God, which isn’t limited to any sect. There are four basic kinds of prayer: Petition (give me something, plz!), Adoration (you’re wonderful), Contrition (I’m sorry), and Thanksgiving (thank you!). Lots of standard prayers can combine multiple types into one prayer (ie, the Our Father), but not all prayer is memorized/recited. But if you do a freeform prayer, it’s important to make sure you don’t just focus on the gimme/gimme/gimme/fix those people, plz! kinds of prayers.
The Creed is pretty much a statement of faith. (“Credo” - “I believe.”) Suppose you have one hundred people who say they all follow Christ— but do they all believe in the same thing? What happens if some people think Christ was a great man, but not God? And others thought that he was God, but not man? And others thought that he’s actually St. Michael? What about those who thought he pretended to die? What about those who thought he pretended to live? Heresies were cropping up from the get-go, and it was important to make sure everyone was on the same page. So the Creed spells out the theological understanding of the basics of what you need to believe. “I believe in God, the Father Almighty, Creator of Heaven and Earth, and in Jesus Christ, his only son, our Lord, who was conceived by the power of the Holy Spirit and born of the Virgin Mary…” etc.
So the Creed was further developed to clarify the basics of faith as additional heresies popped up over time, which is why you have the Apostles’ Creed and the Nicene Creed (325, introduced into the Mass around 500).
We know from Revelation that the saints in heaven offer up the prayers of those on earth. And we know from Jesus in Luke talking about how heaven rejoices over the conversion of one sinner. Do you think that the saints in heaven are offering up a bunch of sealed envelopes, and they don’t know what they’re uniting their prayers to? Or do you think that there’s a Jumbotron in heaven that alerts the saints in heaven, “Hey, one more sinner converted!” so that everyone knows when to rejoice? Or do you think that God allows those in heaven to share in certain graces so that they have the ability to do their work in heaven, after their work on earth was completed?