he was just a pretty simple plot device to flesh out yuffie's story. She actually has a personal beef with Shinra and has lost someone close to her at their hands now. This means minor things like there can be more complex relationship trees developed between her in the future games with people like Barret, Aerith, Vincent because they'll have something in common to share in their hatred towards Shinra. In OG FFVII she's only tagging along to restore Wutai's nationalistic strength and pride, and she's extremely irrelevant to the main story, but now it makes more sense for her to be with the group, and opens up the doors for her character and the wutai (and wutai/avalanche) arc to be expanded upon. Plus she just defeated Nero amd encountered deepground, which means she now has something to talk to Vincent about and trade info. Which she'll want to hear what Vincent has to say because they clearly made it look like Sonon is gonna come back to haunt Yuffie in some fucced up way the way Nero dragged him into the abyss. The set up for that was obvious. Plus tbh, they used Sonon to show Yuffie go from the teenage brat we knew of throughout the whole of FFVII OG to by the end of the DLC she's a bit more wiser, but a bit more damaged, and a bit less of a brat