In every endeavor there are a number of people who are content to stay in the background. They will cede the attention and the spotlight to others and are happy to do their part in anonymity.
While the public personas are the ones that often get the accolades, it is this unknown army that are the backbone of the endeavor. Their hours of effort and sacrifice insure the cause is championed, the effort is effective, and the message is magnified. Often they are never recognized, but the wise leader will make sure to thank them and to make their faithful service known.
As he wraps up his letter to the Roman church, Paul takes time to send greetings to a long list of people who have made his work in the gospel possible by coming alongside him and sacrificing to see him be successful. These are the backbone in the background that made great contributions to the cause of the Gospel.