Or a photo of a young couple playing in a park with their children and the photo is used in an ad for a life insurance company. The models are used to act out some concept or idea of life. The physical requirements, age, size, etc. can vary greatly. Again physical requirements can vary greatly, but usually attractive people are used - although sometimes character models are used. In smaller markets this seems to be a favorite area to start models.
Models stand in front of, or in a store or mall, and hand out free samples of something.
The idea is they want someone attractive that people will be drawn to and will work for just a few hours or a few days. When you are young and starting out this pays better then any regular job and it can really build your confidence around people. Once again attractive people are attention getters. When a trade show comes to town some exhibitors may hire models to hand out literature at their booths. One of the classics examples of this is the auto show.