Why package X is installed in your Debian

When I was looking for alternatives to mount a USB flash drive without sudo, I came across udisks2. To my surprise, this package was already installed on my machine.

I found these two methods to understand why:

$ apt rdepends udisks2 --installed
udisks2
Reverse Depends:
  Depends: gnome-disk-utility (>= 2.7.6)
  Depends: gvfs-daemons
  Recommends: fwupd

$ aptitude why udisks2
i   task-gnome-desktop Depends gnome-core                
i A gnome-core         Depends gnome-disk-utility (>= 46)
i A gnome-disk-utility Depends udisks2 (>= 2.7.6)

The i marker means installed packages.

The A marker means the package was installed automatically.