It's common nowadays that even games as polished as Super Smash Bros. Ultimate (the newest chapter in Nintendo's fighting game series) get day one patches to download. The news comes directly from the series producer, Masahiro Sakurai, who through his column in Japanese magazine Famitsu confirmed the presence of this launch day update.
According to Sakurai, the download is mandatory for those who have the physical version, while the owners of the digital edition will not be affected as it already includes the update. It seems that this update helps solve some problems, including issues with the replay features that are still present in the retail version. At the moment, it's not known how much space this update will take up.