The basic definition of portrait photography is photography of a person or group to capture the personality of the subjects using lighting, backgrounds and posing.
Well, that sounds rather simple.
In one sense, it is simple. You know your purpose when you look at that definition. You want to capture the personality of your subject in a photograph. The problem is that everyone has a bit of a different personality. If you truly want to capture the personality of your subjects, then you need to vary your approach with each portrait session.
Otherwise, your photos end up looking like a high school yearbook.
There are many reasons that people need portraits. Some of them are quite basic.