Carpet pads, also called cushions or underlay, are critical for the appearance and performance of your carpet. A carpet pad -- the spongy sheet of material that is stapled or glued to the underside of your carpet -- protects the carpet from wearing against your bare floor. It also lessens stress on the carpet by absorbing the impact from furniture and from footsteps.
Carpet pads also make your carpet feel softer when you walk on it. Your vacuum cleaner will work more efficiently if your carpet has padding. Padding adds thermal insulation to you carpet and improves its R-value (thermal resistance rating). In addition, padding dampens sound.
Flat rubber has been considered the premium type of carpet pad for a long time. This is because flat rubber is solid and gives good support to the carpet. Flat rubber is long-lasting; when you replace your carpet, you may even be able to keep the padding if you use flat rubber. It's resistant to indentation from furniture and won't bottom out under something heavy. Flat rubber withstands wear and tear, which means it's good for high-traffic areas like hallways and stairs.It can provide adequate cushioning without having to be as thick as some other kinds of carpet pads. It's made of natural rubber, which is a sustainable resource, and it's antimicrobial, so the dirt that collects in the carpet isn't as likely to breed germs.
Flat rubber does have its downside, however. It's not as cheap as foam padding (but keep in mind that foam padding will need to be replaced more often than flat rubber). Finally, flat rubber isn't available at all retail carpet stores because it's usually used for commercial purposes.