Paul's letter to the believers in Rome is more of a theological treatise, being theologically the most important of all Paul's letters, which helped lay the foundation of Christian Theology for generations of future believers. This is particularly because it is his most comprehensive and logical presentation of the gospel which had as its source the Hebrew scriptures, initially the only available source of teaching for it alone contained all the prophecy regarding the coming of the Messiah, and the type of Messiah He would be.
It was Paul's God given ability to interpret those scripture in a way that would be beneficial to both Jews and Gentiles alike that enabled Augustine, an important theologian of the fourth century, to be converted purely by reading Paul's letter to the Roman believers.
The reformation was given birth when Martin Luther discovered the text of Romans and its teaching on justification by faith.
John Wesley's heart was strangely warmed on hearing Luther's preface to this epistle read, causing his conversion by faith in Christ to have a significant bearing on the Wesleyan revival movement resulting in the birth of the Methodist Church.
As letters were hand written and parchment was expensive, Paul's letters contained condensed teaching hence the need for books such as this that expand the text for the benefit of those wanting to properly understand the spiritual teaching and enable the teaching they contain to be put to practical use.