Sara Gruen has the elderly Jacob tell the story to emphasize that it is one person's experience as seen through his eyes and at a distance of many decades. The reader learns that Jacob and Marlena, following their adventures in the circus, stayed together for many years. She was the love of his life, as he was hers.
The experiences that brought them together were so crucial in creating that lifelong bond between them that Jacob vividly remembers every detail. We also learn that Rosie played a vital role in bringing them together, and she continued to be a part of their life after that pivotal episode.
Another reason to have an old person narrate the events is to emphasize the specificity of the time period. It is important to the overall situation in which Jacob found himself as a young man that the country was mired in the Great Depression. His ill fortune at being orphaned combined with the dismal employment prospects in those days help explain how he ended up in the unlikely job of working in the circus.
Because the story is Jacob's alone, however, and Marlena cannot be heard from, we must also wonder how much he is romanticizing or misremembering. Perhaps he is lying. The reader is drawn into the exotic circus world and wants to believe that Jacob is the good guy he seems to be. But no one is alive who could back up his improbable assertions.