I discovered their importance of Isolation Transformers in the school of hard knocks. At that time of this educational experience I had just started working as an apprentice in a television repair shop, and my boss asked me to hook up a our oscilloscope to a TV he was repairing. The moment I attached our oscilloscopes ground wire to the TVs hot chassis I saw a bright flash of light as the TVs fuse blew, and noticed the TV we were repairing was now in worse shape than when it came in. Fortunately my boss allowed me to make a few mistakes, and after this experience I learned why using an Isolation Transformer can be so important.
Thanks to a viewer I was informed that some of the new isolation transformers which use a ground port may not isolated. I suggest you look at this video link by Mr Carlson’s Lab for further reasons as to why this is. https://www.youtube.com/watch?v=XBsQ3sZ45Fk