Paul finally gets to the point of this lengthy letter: I want to visit you on my way to preaching the gospel in Spain. Spain was a significant player in the Roman empire and produced several generals, poets, writers and other leaders in the 1st century. Paul's hope is that he may go and preach where "no one has preached before". His desire is that he will be able to visit the church in Rome, preach, teach, etc. take a collection and go on his way.
This is an important insight into the life of an Apostle. Apostles were the sent ones and they were not sent to preach in established faith communities but to begin new faith communities. The apostolic work in the modern world is closer to the work of a church planter. Planters, using many different strategies, begin faith communities where no such faith communities exist. However, the modern church planter is often working (especially in the United States) where the gospel is regularly preached and where other churches already exist.
Among my Uganda friends, where there are villages that do not have churches and tribal groups that have yet to be reached with the gospel. The church planters are often small groups from one church that discover and "unreached" area and send a team to preach, teach and organize new Christian communities. This type of planting behavior is much more common where there is a majority of "first generation" Christians and, I believe, is closer to the first century apostolic activity.
Paul's hope in chapter 15 is illuminated by history. In Romans 15:25, Paul mentions that he is taking a collection for the poor from Macedonia to Jerusalem and then hopes to make his way to Rome after that. We know from the Book of Acts that Paul was arrested on his mission to Jerusalem. He was imprisoned for several years and eventually takes his appeal to the Emperor. When Paul finally arrives in Rome he arrives in chains and under house arrest. History does not record if Paul ever makes it to Spain. Tradition holds that Paul was martyred in Rome following his imprisonment.