When are "User Privileges" relevant?

I notice that many of the user privileges in "Users and Groups" are not ticked on my user, but I still have access to the items they specify. For example, I'm constantly using VirtualBox for virtualisation, I can connect to wireless and wired networks and I can plug in and use any audio device.

When are these privileges honoured? How useful are they?

There is no one answer to this question, as each permission has it's own effects. Most of that "privileges" means that you are a member of some group. This will usually let you access to files owner by that group. Remember that in most unix-like systems everything is a file.

Now, one answer:

"Use VirtualBox virtualization solution" likely means that you are member of the vboxusers group. You will need to be member of this group to be able to use usb devices on VirtualBox. If you are not member of this groups, all the devices on the USB menu on VirtualBox should be greyed.

Most of these options are "legacy" and have been replaced by tools like ConsoleKit and PolicyKit for managing what users are allowed to do and when.