I went to
/index.php?module=UsersManager&action=index&idSite=1&period=day&date=yesterday on a Matomo install with only one site. Then the screen looks like this:
It appears as if the button is clickable but when you click on it, nothing happens. It feels like it's buggy/broken. As the user you don't really know that this is a site selector and that it only works if there's more than 1 site.
Not sure if the same issue exists maybe also in other places.