True lab supplies look like batteries. You can stack them on other supplies, make negative supplies, positive ground them, negative ground them, etc, etc. They are "floating" with no reference to earth ground (unless you strap them with that little earth strap). There are limits to all this fun related to the supply's construction, you're usually safe to do anything up to about 100V.
Usually it doesn't matter but when you start hooking up other earth grounded stuff, or other floating stuff, you get ground loops. PC's are a notorious ground loop creator. When you connect two things that are "grounded" to different ground potentials, currents will flow in the ground connections where it shouldn't. If two things are not plugged into the very same outlet, the ground potentials between their green wires could be quite different. (volts).
One of the better demos of this was a video that Dave Jones did called "How to Blow Up an Oscilloscope" or similar. Its goes though the issue from a different and more expensive vantage point.