In the previous five letters Paul wrote we see him dealing with churches he started and problems that they were facing. They are all “case specific” letters. Romans is something new. Paul is writing to a church that he did not start; he hasn’t even been there yet. Therefore, Romans is a careful description of the Gospel message, not formulated out of need or in urgency, but clearly thought out and by way of an introduction.
(That being said it is grouped in with Galatians and the Corinthian letters as a part of the “Big Four.” These four letters share more than mere length; they all deal with similar themes. Mainly, Paul defends “his” Gospel message against the influence of the Jewish teachers that were trying to convince his churches to be more Jewish. For instance the words “circumcision” and “law” occur frequently in these four letters and hardly at all elsewhere in Paul’s writing.)
Whereas Galatians and (even more) the Corinthian letters can be read as a “how to” for church planting and church life; Romans is more about the message itself—that understanding that ties a church wherever it may arise to all other groups of believers in the world and throughout time. “It is the power of God for salvation to everyone who believes.”