It can be one thing to have a vision for how a room will turn out, and another matter entirely as to how it turns out. Looking at pics of decorated rooms can ensure that the person is on track with this vision.
When people see pics of decorated rooms, they often get a feel for what the decorating trends are. That is important to people who want their homes to look modern.
Others like pics of decorated rooms to know how to make their rooms have a certain ambiance. For example, people might like the country western theme or Tex Mex version but not know how to execute that. Pics of decorated rooms will display items of decor that give the room its theme.
When looking at pics of decorated rooms, keep in mind how long you will be at that place. If it is just a rental, then a person will want decor that transports easily.