Paul’s intended trip to Rome, a journey that he purposed to eventually take him to Spain (Rom 15:22-29), had two basic objectives. The first was to enjoy fellowship with the believers in Rome (Rom 1:11-12), and the second was to preach the gospel to those who were in Rome (Rom 1:13, 15).
Paul had, at the time of his writing, never visited the Roman church in person (Rom 1:13; 15:22-29). He thus expands his salutation so as to include a creedal summary of the gospel and his apostolic calling. Presumably Paul’s intent here was to establish credibility with the recipients of his letter, whom he had not yet met with in person.