Isolation tranny is useful to reduce high frequency noise on mains, to increase magnetic reservoir, and in balanced isolators to provide floating ground to eliminate ground hum. But like any large transformer, they can make their own mechanical hum so they sometimes require mechanical isolation themselves if used by a golden-ear in a quiet room. Bryston makes single ended isolators and Equitech makes balanced iso-trannies if you want to research.
I had two memorable experiences with Equitech Q1.5 balanced isolation transformer. First, I used it with pair of 100w tube monoblock amps. The tranny's floating ground eliminated any hum and made the background noise inaudible. But more noticeable was the improvement in amp dynamic performance with the enormous magnetic reserve added to the puny power amp's own power tranny. Huge improvement overall as the amps really came alive. The second experience, around that same time and as I was becoming an balanced line conditioner evangelist, a friend tried the Equitech with his DIY SS 100w integrated amp in my system. It had excellent ground design, already dead silent noise floor, and an adequately sized power transformer onboard. We heard no improvement at all with the isolation tranny vs the wall outlet. He said "told ya so" and I learned a lesson. No "magic tweak" works in every situation. But the Equitech was one of only two power conditioning devices I have tried over the years that actually made any kind of positive difference in my system. The other was a regenerator, which did make his integrated amp sound much better (and he agreed) but costs a lot more than any passive conditioner.