Why Xi would want to participate in something like this is puzzling. Unlike Putin, he certainly doesn't need summits for summits' sake, least of all with Trump. In addition, South Korea is a really weird venue for a meetup - given that it's not neutral ground. There's also no clear agenda, so I suppose the only reasonable explanation is that Xi expects to get something for free; something valuable enough that he'd want to look past the questionable optics at play here.