Side Covers.

Why do computers have side covers ?
You know, those plastic or metal plates that cover thier sides, yes, the ones that you take out, when you screw in your video and sound cards, yes, those plates that sit there, next to your computer, sitting, as if they’re waiting for you to close your computer with them, as if they’re ment to protect your computer from something, or to cover it from dust and dirt. Why do you have them ? What use are they ?