1- the AC power your utility supplies is not stable and not clean;
2- it must have capability to supply at least double (more is better) the current your system draws during listening.
How do you know if the power supplied is stable or not?
I use a vintage analog meter made by WIZ (Power Line Monitor) whose needle will tell me how the power is being supplied.
This one here: https://www.ebay.com/itm/RCA-WV-120A-AC-POWER-LINE-MONITOR-METER-Vintage-works-perfectly/15301646919...
When the power supplied from your wall socket is very stable, there is no detectable benefit by using a power conditioner.
Important: the power conditioner I use not only supplies very stable AC voltage (1% regulation) but also have a very good RFI eliminator.
How do I know whether there is lots of noise coming from the power line? I use a noise sniffer like this one made by Audio Prism.
http://www.audioprism.com/noisesniffer.html
Regarding the capability of the power conditioner, in my case I use one capable of supplying 50A while my system draws from 8 to 13A while listening.
Also, my electricity is supplied by a 3 phase system and I use one phase to plug refrigerator, microwave, washing machine etc.
The other phase I use for air conditioning and the third phase for my audio system and lightning which are all using LEDs now.
How do I select the phase for my audio system? I plug the analog meter and watch for some time which one is more stable.
So, the question is do I notice any improvement by inserting the power conditioner in my system? The truth is, I don't, since the power I get from the utility is very stable.
But I leave it there anyway since it protects my system when there is an over voltage situation (very rare) or under voltage situation (also very rare).
Also it cleans the power from noise coming from the power line and that is good.