Maybe if you live in an area with very modern all underground power company lines(which are unlikely to pick up RF), a utility that never does anything but top notch maintenance / service, and a home where all the "high draw" appliances are well isolated from the A/V system the improvements when switching to a nice well regulated power line conditioner will be less noticeable than in an area with above ground lines, poorly maintained utility feeds and typical appliances but I swear you will still see / hear the difference. Believe I am as skeptical as anyone about any sort of "audio snake oil" but the reality is that MOST homes do have some real problems with their electricity and well designed conditioners clean that up. The fact is MANY a/v stores have CRAPPY power too and the smarter dealers invest in their "house" too so that they can demonstrate equipment at its "least degraded" level of performance. In contrast crappy big box stores often have crummy looking / sounding equipment even if it from a "high quality" manufacturer.

When TVs do not get good juice you honestly will see random "sparklies" or noise. When sources do not get good juice the signal can suffer from things like insufficient gain, failing to get a nice consisten lock on HDMI signals, increased local RF / EMF noise and general loss of dynamic range. When amps do not get good juice the noise floor is often raised significantly, sometimes because the "bad" power has a whole lot more RF than it should AND/OR because the amp working extra hard to try to amplify crud instead of the "good" input signal...

The safety / protection / device longevity is pretty much worth the cost of devices alone and even if you are not a super critical viewer / listener I would strongly recommend good line conditioners for that reason.