With some plasma/kde developed package updates, several configuration procedures, settings, and/or values change in a way that is not backwards compatible. This often produces such problems in Plasma/KDE sessions, because, while the new packages are installed on the system (system folders/files), the already existing configurations (folders, files) in the users’ folders are not automatically updated. Then, when you login after an update (and reboot), the local files do not include the correct settings/values that the new packages expect, leading to a broken session.
Thus, deleting the defective local configuration file, triggers plasma to create a new, clean file from a system template, which contains all the correct directives, settings, values. Of course, you need to re-customize any lost custom configuration again.
FWIW, plasma devs could have taken care of this bad situation, if they wanted, or if they could (although, I am pretty sure they have enough skills to do it 
).
For example, OTOH, Gnome has such a mechanism (a bash script file) that handles differences between old and new versions, even if this is only for one simple task. The idea is exactly the same.
I would have opened such a bug report upstream KDE, but I hate to point out such an obvious and so widely known problem with plasma updates, towards a team that is so busy developing a trillion of features, that forgets to make their users’ (customers’) lives a little easier (saving them from their development insufficiencies).
Sorry KDE lovers for the harsh comment, but this is from my personal experience. 
