I was convinced of the benefits of power conditioning years ago at the beginning of my equipment journey. I was auditioning some Numan Loudspeakers -- long since departed the highend. The designer was extolling their virtues and asked me if I wanted to hear them sound at their very best.
He switched on an MIT box. This unnasuming, utilitarian piece of kit instantly improved the sound - veils were lifted. I thought it was a different amplifier, readying for the hard sell. No, a power conditioner. And a damn fine one.
Over the years, I've had more than a few to play with. Some, much like the Audio Power Industries behemoth that's been in my system for over 20 years, do a very good job of cutting out the hash and other nasties. A couple, however, were unmitigated failures. The primary failure was a loss of power and musical energy. Robbing Peter and never paying Paul.