In your country, only dentists may wear this type of masks. But here in Japan, you'll see a lot of people with a mask. When you see someone like this on a warm day in spring, she is likely to be allergic to pollen. One out of five to ten Japanese are said to have a pollen allergy. If you see someone like this coughing, he is likely to have flu.
People wear masks to prevent flu, too.
If you find someone like this in an elementary school, he is very much likely to be in charge of serving school lunch. Children take turns at serving lunch for educational purposes.