Why ? RF nasties. There is a good bet you suffer from that symptom, either audio or video, probably both. Hell, whose system is free from RF/EMI to some degree ? Even digital components within a system affect each other. Digital grunge and line hash is omnipresent.
For a power cord, fixed or IEC, check out the Highwire Power Wrap at amusicdirect.com Cost is low (about $25.) and they will beat out snap-on ferrites any day. They are a foot long coil with a blue plastic covering. You inter-twine them around you cord. Then you "tune" them (basically centered on the cord, give or take a few inches, maybe) by sliding them for the best effect. Voodoo? Not in my case, but YMMV. But they have a long following and I am just another happy customer. I also use one on my Yam DSP-A1 amps fixed cord.
I have used both, and the Highwire is the most efficient. Put one on my fixed cord for my RPTV and , bingo, no more flicker, guaze-like "spider-web" lingering in the background. I already rid myself of the faint snow via a dedicated circuit and a Jon Risch Line Conditioner. I also run my CATV coax through a Monster HTS 2000, and use a Jenson CATV ground loop isolator. That is the only use for the Monster HTS 2000.
The Highwire works. But you must be aware that RF/EMI has many ways to enter your system. I have gone to great lengths to combat RF, and the Highwire was the last piece that made CATV look as solid as a DVD image. Granted, some CATV channels just suck in terms of quality, and there is nothing one can do to help the issue.
As far as audio, a blacker background comes to mind. Other benefits depend on the merits of the system.... etc
PS -- Radio Shack does indeed sell snap on RF chokes (red plastic rectangles encasing a ferrite sandwich), and in 2 sizes.
I added ferrite chokes from Audioquest to the power cord of my DVD player and my CD player and notice vast improvement in the sound quality (better imaging, deeper bass). When replaying CD I have had for year I hear things in the music that I didn't know was there.
Maybe it's because I own budget level equipment CD player is a HK 710 and the DVD is a RCA 5220, is why it is so noticeable. This isn't voodoo, it works.