Ubuntu – the use of *-doc packages

If you opt in, popularity-contest reports usage statistics back to Ubuntu, but it could also be used for purposes like yours. From it's man page:

The popularity-contest command gathers information about Debian pack‐ ages installed on the system, and prints the name of the most recently used executable program in that package as well as its last-accessed time (atime) and last-attribute-changed time (ctime) to stdout.

The key word there is executable. So you'll probably get some false hits for data packages, etc.

So, for example, popularity-contest | grep '<OLD>' should give you a list of packages that have not been used for more than three months.

popcon-largest-unused gives you a list of unused package sorted by size.

The procedure I use is:
sudo apt-get install htop apt-file
sudo apt-file update
htop
f2 for setup
Navigate with the arrow keys Down Arrow,Right Arrow,Space to toggle tree view, Esc to get back to menu(because you can't send F10 to the gnome-terminal easily). You can also do this with the mouse.

Now you get more informative information about exactly how each process is launched and other files it might load. So oneconf-service for example.

man oneconf-service and man oneconf yield nothing. apt-cache search oneconf-service yields nothing.

apt-file search oneconf-service yields oneconf: /usr/share/oneconf/oneconf-service
apt-cache search oneconf yields oneconf - synchronize your configuration data over the network

To go back to your particular ones in question:
for apt-file search gvfs-gphoto2-volume-monitor you get:

gvfs-backends: /usr/lib/gvfs/gvfs-gphoto2-volume-monitor
gvfs-dbg: /usr/lib/debug/usr/lib/gvfs/gvfs-gphoto2-volume-monitor

And apt-cache search gvfs-backends yields:
gvfs-backends - userspace virtual filesystem - backends

apt-file search 'rtkit-daemon' yields:

rtkit: /lib/systemd/system/rtkit-daemon.service
rtkit: /usr/lib/rtkit/rtkit-daemon

apt-cache search rtkit yields:
rtkit - Realtime Policy and Watchdog Daemon

Edit to address comment: I recommend htop because you can see other loaded modules not just the process name. If something launches in a funny way, the output of ps/process name may not show you something horribly useful.

Apt-file can find things that are named differently than the package name and aren't in the package description. It lets you track any file installed by a package back to the package.

userspace users can control it virtual filesystem not exactly a filesystem, but it acts like one - backends the part you don't see. If you check in htop, it talks over dbus(it shows in loaded modules). This makes me think it lets users manipulate filesystems/mounts/devices over dbus (a common way to grant super-user privileges without requiring sudo and using package-kit instead). So when you combine htop output, knowing a common use for dbus, and the description, it becomes a lot more clear.

Watchdog a process that monitors another to make sure it doesn't run out of control daemon something that runs as a service. So it's a process that monitors other processes to make sure scheduling doesn't get out of control.