Passive constructions omit the agent of the action. In many situations, the agent is clearly recognized despite its not being explicitly stated, as it would be in an active construction, or the agent is not relevant, so that its omission results in no loss of clarity:
At this stage in the experiment, salicylic acid was introduced.
This bicycle has been ridden so much that it already needs new tires.
The convicted criminal was sent to jail.
However, in many other circumstances, a passive construction with an implicit agent does result in a loss of clarity or in incompleteness, because a piece of information is missing from the statement.
The leak in the toxic waste storage tank was never repaired.
Chemical Waste Mismanagement Inc never repaired the leak in the toxic waste storage tank.
The veterinarian was told to give the horse a shot of noradrenaline 30 minutes before the big race.
The organized crime boss told the veterinarian to give the horse a shot of noradrenaline 30 minutes before the big race.