Plasma

Application version: to know if our users are up to date
Compiler information: to know if issues in non-mainstream compilers are relevant
Platform information: to know how popular the Wayland session is compared to X11
Qt version: To how different versions of Qt and Plasma interrelate
Usage time: to find out how many users are using the software heavily vs in passing
OpenGL version and vendor information: to learn about our users' hardware; can we trust users to have proper 3D acceleration? Which drivers should be testing? Do people prefer open drivers?
Locale information: To find out if people from different locales are struggling to use Plasma. Are we not getting bug reports because they just don't even consider using it?
Screen parameters: to find out how common multi-screen usage is in Plasma
Panel Count (Plasma Shell): to find out how common it is for people to customize their Panels
Application Source Name (Plasma Discover): to find out where users prefer to get their software from