When you look at the bottom of your shoes before entering your house, they may not appear to be all that dirty. If you've been walking on cement sidewalks and asphalt, then there shouldn't be many visual particles on your shoes. Looks, however, can be deceiving. In fact, a recent study found the age-old "five second rule" (the one that suggests it's okay to eat food that's fallen on the floor so long as you pick it up within five seconds and your floor looks clean) is false. An item that hits a floor for even a fraction of a second is usually contaminated with something, so whether it looks like it or not, if you eat something that has been on the floor, no matter how briefly, you are ingesting some unwanted germs and bacteria. In the same right, even if your shoes look clean because you have been walking on concrete or pavement, there is undoubtedly some bacteria on them that you would probably rather not have in your home.
Research done at the University of Arizona found that, if a person wearing shoes carrying a bacteria walked onto clean floors, that bacteria would contaminate said floors at least 90 percent of the time. So even if you only walk through your home with dirty shoes ten times a month, nine of those times, you will effectively transfer some bacteria to your floors. Placing a doormat at your entryway and requiring people to thoroughly wipe their shoes off before coming in could limit some of the bacteria that enters your home.
The germs you bring in on your feet don't only stay on the floor. They usually contain tiny particles capable of traveling upwards, contaminating the air you breathe. Depending on the material of your doormat, it could trap those particles and prevent them from rising and contaminating the air quality in your home. After considering these factors, one can see how having a doormat isn't just a style choice, but also a health-related one.