The isolation transformer can remove ground loop issues, or if some of your AV equipment has a switching power supply with that annoying class Y cap and poor hipot isolation. I used an isolation transformer to power my old Sony 757 HiFi VHS from ~1989. It removed a nasty 60Hz humm/buzz sitting at around ~ -82db making the noise floor below -96db, slightly better than the VCR's advertised noise floor. This was heard clearly and the 60Hz hum was eliminated with that transformer.
I could not use such a transformer on my PC feeding the VCR audio inputs since my isolation transformer was only capable of ~75 watts and my PC was over 300 watts at the time. There was no point in isolation my amp as it already had an old fashioned linear transformer power supply. Yes, my RCA cables had heavy-duty thick copper braided shields.
So, yes, if your situation creates such a problem, a 'strategically' placed isolation transformer can solve some specific problems. If you do not have an audible fundamental multiple of mains entering your signal path due to a limitation in the mixing of audio equipment, an isolation transformer wont do anything for you.
My isolation transformer cost me ~25$ in total. It was 2 back-back transformers, 120v in to 48v, then a second identical one took the 48v in and gave me back 120v. They were the old split-bobbin style transformers meaning an extra hipot isolation. There was no regulation or any filtering.