Having some idea about the structure of the letter going in is a good idea.
After all, we aren't the first recipients/readers. So there's already a history of reading this item.
I don't think the letter is in the slightest off the cuff, or filled with "digressions", or the like. It exhibits a highly organized structure, and a strong logical flavor (as someone said, the question-response format is an important element, which Paul uses to prosecute his overall case).
Paul is writing to a church he has never been to see, and had no part in planting (indeed, no apostle had been). He desires to go (he knows some of the Christians who have relocated to Rome), because he does wish to "establish" them--technical language--which would include the "imparting some spiritual gift" to them (extraordinary gifts being signs of the apostles).
And this blessing of them has a further motive (one he doesn't openly state until the end of the letter). He wants "missionary support" from them. He want a new base of operations, from which he plans to extend his gospel labors to Spain.
So, what else is needful? He needs to give out the content of his gospel, that is, the content of the message he preaches to the lost. If YOU are being asked to support a missionary, don't YOU want to know what sort of preaching you are going to be supporting?
So, starting at Rom.1:16-17, we have the gospel-theme of the letter presented. Rom.1:18 begins the presentation of the "good news" by first presenting the "bad news," that is, "wrath revealed." This is followed after 2 chapters, at 3:20, with "righteousness revealed." So far, the general structure of the first 11 chapters.
Ch.12 begins the "practical" portion (a general pattern we see frequently in the letters: doctrine-application). All we're talking about is basically the gospel-realities worked out in the details of actual issues in people's lives. Paul may be addressing general problems, or specific problems in their church, of which he has heard news.
Bottom line, this letter shows ample evidence of much prior thought, and careful planning and organization, prior to a single word put to ink-on-parchment. Prior knowledge of something of the structure will be of service to us, who do not have the benefit of immediate relation to the material, or access through the original language.