Paul's position can be justified only as resting in God, and so only can his words be regarded as at all credible, for they are as incapable of direct apprehension as is God Himself. For this reason he dares to approach others and to demand a hearing without fear either of exalting himself or of approximating too closely to his audience. He appeals only to the authority of God. This is the ground of his authority. There is no other.Karl Barth, "The Epistle to the Romans," pg 28. vv 1-2. CH 1.
Paul hasn't yet been to Rome that we know of, and the church has already been established. He is eager to stress his credibility and his calling, of that towards the Gentiles (Acts 18:6; Eph 3:1-3). His calling is manifest in his most comprehensive letter (that we know of), and he is eager to not only go to the symbolic "center of the (known) world" but he is also interested in possibly going further into Spain (Rom 15:24, 28). Much of the early church believes Paul journeyed to Spain, and we seem to have manuscripts that suggest it, such as 1 Clement 5. We don't know if Paul made it there, though Clement and other manuscripts seem to suggest that he did.
Either way, Paul claiming the authority of Israel's God and taking this to the already established (fledgling?) church of Rome was a strong example not only of a miracle (given the transportation and fiances required for such a letter, delivered by a woman no less [Rom 16:1-2 indicates Phoebe carried the letter and implies a possible teaching role as she explained the words Paul had written, though Barth accepts the poorly mistranslated 'Junias' and indicates that he is less likely to accept the role of female apostles in a footnote, "Epistles to the Romans" pg 535-36]). For strong textual/exegetical cases for Junia as an apostle, see footnote.
Either way, this commentary is fascinating (save for the unfortunate aspect of "Junias"). Especially for those interested in a more theological treatment of Romans.
For a bit of background, enjoy it here.
Eldon J. Epp, "Junia" (the entire book is a textual treat).
Richard Bauckham, "Gospel Women" pgs. 166-169.
Philip Payne, "Man and Woman, One in Christ" pgs. 65-7.
Cranfield, "Romans" Book 2, pgs. 788-90.
Dunn, "Romans 9-16" pg. 894.