The objective of studying Active and Passive voice is to understand the focus the importance of the doer or the action performed in the sentence.
Active voice is used when the doer of the action is to be focused.
Passive voice is used when action is more important than the doer.
Monkeys plucked the flowers. Active voice - focus is on monkeys.
Flowers were plucked by the monkeys. Passive voice- focus is plucking of flowers and doer is secondary.