In the social sciences and related fields, people are most likely to use the passive voice when they do not want to call attention to who or what has done a given action. When the passive voice is used, there is no need to specify who has acted in a given situation. This can be useful to people who are writing in the social sciences, history, and other such fields.
One reason to use the passive voice is if you are not sure what has caused a certain situation and you do not want to advertise that fact. For example, let us say that you know that the crime rate in a certain city has gone down. While you know this fact, you are not sure how to account for it. Therefore, you can say things like “the crime rate in City X was cut in half during the tenure of Mayor Y.” By saying this, you sort of imply that Mayor Y might have had something to do with it, but you do not actually come out and say so.
Another reason to use the passive voice is if you know who or what caused a situation but you do not want to emphasize that point. For example, you might say “the atomic bombs were dropped on Japan, thus ending WWII.” This allows you to avoid specifying that the US dropped the bombs. Since some people criticize the US for doing this, the use of the passive voice allows a writer to gloss over the fact that it was the US that dropped the bombs.
If you provide a more specific context, we may be able to give you a more precise answer as to why the passive voice is being used.