Aside from plain orneryness, I picked Linux long ago because of KDE, and KDE because of Kontact. And because it was long ago, I still pull my mail off the ISP's server and dump it into a local Maildir folder, my contacts are still in a vCard file, my calendar is still in a vCalendar file.
The "Cascade of Attention-Deficit Teenagers" development process (Google it) made this harder with each KDE release. For a long time I was willing to edit RC files to trick it into using boring text files in my home directory instead of shiny new formats in hidden directories, but the CADT Team finally produced a release where the shiny desktop search daemon rendered the maildir folders unusable if a mail filter tried to move mail out of the inbox. After three rebuilds I gave up and switched to Evolution.
Evolution is mostly adequate, but I just spent 5 hours composing a carefully argued response to a proposal, and Evolution threw it away when I tried to apply the TT style to a word in a bulleted list.
So, it you wanted PIM functions on an Ubuntu 17.10 laptop, and you wanted to keep your files locally, and you wanted to know where all the relevant files were so you could back them up, what would you use?