Regular users love a "Show Desktop" button in any OS they're using. KDE Plasma offers one in the form of the "Show Desktop" widget, but does not add it by default. Kubuntu should! The best place for it is on the far right side of the default panel, so that when the panel is locked, the Show Desktop button includes the bottom-right-most pixel, making it easy to access by flinging your mouse in that direction and clicking (AKA Fitt's Law).
(Upstream task to make this this default is tracked in T7917: Show Desktop widget should be present by default in new installs)