when active voice is to be used

use the active voice when the actor's identity is important.

active passive voice

examples
Active :1) Ali did it.
Active :2)The butler did it.
passive:3)It was done by Ali.
passive:4)It was done by the butler.

The first two sentences are stronger,they sound like an accusation.the 3rd and 4th sentences are weaker,they sound like 'Ali' or 'The butler' did nothing more sinister than a load of laundry.

.Use the active voice when you want to be direct and emphatic.

Active:Management expects the staff to follow the rules.
Passive :It is expected by management that rules will be followed.
In this example ,the active voice delivers the message more actively.
In this exampletheactive voicedelivers the message more actively.