Contrary to what you thought, a carpet isn't really enough to protect the sheet metal flooring of your car. During winter for example, especially if you live in areas where heavy snow and frequent rainfall is a norm, liquid may soak your carpet, seep to the vehicle floor, and cause it to corrode. And even if you live in an area with a dry climate, dust and dirt could push you to have your carpet cleaned and replaced every so often. Considering the price of carpets these days, such is very impractical.
But a floor mat or floor liner can keep that from happening. Instead of having your carpet cleaned all the time, you can just remove the mat from the vehicle and then scrub the dirt off with water and soap. Floor mats are indispensable for your vehicle whether it's dust or rain you see in your place. There are several types of floor liners to suit your needs. There's a carpeted floor mat for those who have dry climates, and an all-vinyl or heavy-duty floor mat for those who live in the snowy parts of the country.
There are also several designs and colors of floor mats available. In fact, you can have them customized to complement the feel and look of your vehicle. You should always choose well, however, by balancing aesthetics and practicality.