While Ubuntu does have a wide selection of software in their official apt repositories, as well as what is available in the official (but unsupported) Backports repository, providing packages for newer KDE software versions (as well as some major applications) is often beyond the scope of these official software channels.
This is why Kubuntu use the Launchpad PPA system for their KDE technology upgrades.
This is where all your security fixes come from.
Normal package updates come from here. Note that you will not always get every new KDE software version.
This is the official Ubuntu Backports repository. this unsupported channel mostly contains individual packages "backported' from a newer Ubuntu release, but will usually provide a minor KDE software "point" release if it provides significant bug fixes and stability.
Updates for Kubuntu releases which are due to go to Ubuntu Updates. Mostly KDE point releases.
Example: Kubuntu Wily 15.10 users were able to test out upgrading from Plasma 5.4 1 to 5.4.2 before those packages were moved to Ubuntu Updates.
Use Case:
Open your Terminal (Konsole) to add:
sudo add-apt-repository ppa:kubuntu-ppa/ppa
sudo apt update && sudo apt full-upgrade -y
Backports of new major versions of Plasma, Frameworks and major KDE apps for Kubuntu which are not yet tested enough to go to Ubuntu Backports.
Example: Wily users were able to try out upgrading Kubuntu Wily's Plasma from 5.4.x to 5.5.0 without having to wait for Xenial's release.
Use Case:
Open your Terminal (Konsole) to add:
sudo add-apt-repository ppa:kubuntu-ppa/backports
sudo apt update && sudo apt full-upgrade -y
KDE Community Wiki 