It’s because it has offline updates enabled in Discover settings, its not a distro thing rather that Gnome and KDE have that in their software centers as a setting, and Fedora enables it by default, but it can be disabled.
It’s to make your system more stable because no packages get moved or updated during a running system causing unexpected behavior, and you also don’t have to reboot when it tells you, it’s more just a reminder that updates are waiting to be applied when you do, they could really word that better.