Passive voice is used when the speaker wants to shift the focus from the Agent to the Action or to switch the importance of the subject and the object. Maybe you don't know the Agent or the Agent is less important (in the sentence) than the Action itself. It can also be used to create a more objective tone. http://owl.english.purdue.edu/owl/owlprint/539/ http://international.ouc.bc.ca/chalkntalk/whypassive/ http://uwc.utexas.edu/handouts/passive http://owl.english.purdue.edu/owl/owlprint/539/

The speaker generally has no intention. He is usually required to use the passive under certain circumstances.