What I mean by symbol here is anything in a photograph that represents, in a viewer's mind, some larger thing. A cross might stand in, or evoke, a collection of ideas about religion, for instance.
People, especially people with specific collections of accessories, are powerful symbols. A man wearing a pinstriped suit, talking on a cell phone, could be very evocative. For some viewers he might represent banking, or industry. For another viewer the man might represent Dad. Most viewers have known many people in their lives, and have seen many photographs of people. All of this experience is in play when the viewer sees a person in a photograph.
Since people are things we're intimately familiar with, we're more likely to compare the image of a person with other people's images we have stored away in our minds. A viewer with expertise in bulldozers would do the same with an image of a bulldozer. While few of us are experts on bulldozers, most of us are experts on people.
Pictures with people in them are potentially pretty powerful stuff, but potentially lead to a wider range of interpretations of your image.