Outside of a few co-op titles and maybe some sports games it isn’t clear why you might want to use more than four controllers at once? It could be that Microsoft is planning for the future by offering support to handle eight controller connections and these controller connections could be used perhaps by other devices in the future? No way to tell.
While the PS3 supported the use of seven controllers, Microsoft has one upped that with support for eight on the Xbox One, but the question is if it actually matters. We don’t think so, but you never know what the future might hold.