I hope to see you in passing as I go to Spain, and to be helped on my journey there by you, once I have enjoyed your company for a while.
Romans 15:24 ESV

Most maritime trading routes between the Eastern and Western Empires passed through Ostia Antica, the major port nearest Rome. Having determined to take the Gospel to Spain, a stop in Rome wasn’t out of his way. In fact, it was probably unavoidable, and he wasn’t going to waste the opportunity to visit with the Roman assembly and possibly receive financial assistance from them to aid his mission.

Such journeys also usually took weeks or months to complete. A journey from Tyre to the Spanish Mediterranean coast would likely make a half-dozen or more stops along the way, not only to replenish supplies, but to engage in trade. The ship could be in each port for a few days to weeks--longer in larger ports like Ostia Antica--and passengers might wait or look for another vessel for the next leg of the journey.