For St. Thomas, the angels, besides being a specific theme to which he dedicates numerous questions, are above all something like an integral part of the global Christian mystery.
From the point of Christian life, what is truly "useful" is to accept God's plan as God designed it, because what God does is always full of wisdom and instills wisdom in those who accept it docilely.
St. Thomas dedicated to the first chapter of the letter to the Hebros, because that is where the relationship of the angels with Christ appears most clearly.
The answers are: 3.- He wants to connect his philosophy with the Church, and 4.- He was interested in the chain of the divine.