Power conditioner or not?

I use a regenorator by PS for my front end gear. Having said that, I cannot think of any preamp, amp designers who use any type of power conditioners when designing their gear. Having spoken to them not one says they are needed to make a proper designed power supply function better. I like the PS for keeping my voltage steady at 120, perfect wave form minus any line noise. It does work because when I run my vacuum the meters show the incoming distortion and the improvement and they both go through the roof with vacuum runing. So the PS Audio does so what they claim it does. I used balanced units and transformer units and I have kept the PS Audio unit. 

My system has all-digital sources (Roon server & disk transport) with a DAC that is separate from the preamp.  I have found that conditioning the power improves the performance of my DACs, most notably tightening up the bass and improving the stereo image and soundstage.  Each time I've improved my conditioner, the sound quality improvement is immediately better.  I can't speak to whether power conditioners would improve a system with analog sources, but I'd say "yes" to those with digital sources.

How do power conditioners work? 

I assume they try to filter out variations of voltage but it's AC in and AC out.

They also must have a circuit breaker to try to block spikes in voltage caused by lightning or other big shocks. 

I am confident that most quality equipment designs in beefy transformers,  rectifiers and filter capacitors that handle most anything they are likely to encounter.