So you've just bought a high-definition home theatre system. You get it home, plug it in with the included cables, and you're set, right? Not so fast. If you're using poor-quality cables, like the ones that come in the box with most audio/video gear, you could be throttling your system's performance.
Think of it this way: If you owned a high-performance sports car, you wouldn't outfit it with low-end tires or you'd get insufficient traction, poor handling, and increased road noise. You'd lose all the benefits of buying a high-performance car in the first place. In the same way, low-quality cables can rob you of the performance you paid for when you bought your system.
Poorly made cables can allow noise and interference to compromise the signals coming from your source components, resulting in a sub-par listening or viewing experience. They generally won't give you the best-quality connection, and often aren't built to last. And of course some free cables may simply be too long or too short for your setup, making them less than ideal.
Replacing free "in-the-box" cables with higher-quality ones, or buying a step or two up from the cheapest quality cable you can find, can really make a difference when you're building a nice audio/video system. You'll enjoy more realistic sound and a clearer picture when your system isn't hampered by weak links.
(Source: Kristen Schaub of Crutchfield)