The 'normal' from is the active. You use the passive voice mainly when you don't know who did the action: My bag was stolen. or when it's obvious or unimportant: The package was sent. The passive puts the focus on the action itself, or on the result of the action, and not on the person who did it.

You rarely NEED to use the passive voice. (As Su.Ki says, it is needed if you don't know who performed the action). Usually, it is a matter of style. Style authorities Strunk and White recommend using the active voice whenever possible. "The active voice is usually more direct and vigorous than the passive... This is true not only in narrative principally concerned with action, but in writing of any kind." Scientists use the passive voice to avoid using the pronoun "I", which is thought to be egotistical. Instead of writing "I heated the solution to 37 degrees," scientists write "The solution was heated to 37 degrees." Administrators and politicians use the passive voice evasively, to avoid blame. Instead of "I made a mistake," they say "Mistakes were made."