Why is the agent omitted on some Passive Voice sentences?

There are three possible situations were the agent could be omitted on this kind of sentences, and are the following:

• When the agent is unimportant.
• When it is obvious.
• When it's unknown.

When the agent is unimportant.

We use this when the doer of the action (agent) is not important to mention and also is not going to give us important information about the action that was performed.

Some examples of this situation are the following:

    1.  Two cars were used to hit the barrier. 

                Were used by whom?

     2.  10.000 guns were made to supply the army.

                Were made by which company?

     3.  That headphones were used to hear private conversations.

                Who used to use that headphones?

     4.  The company was directed using a new strategy.

                Who was directing the company?

     5.  Military watches were developed to help soldiers on different tasks.

                Which company developed the military watches? 

When the agent is obvious.

We omit the agent when we know who performed the action. In this case, it's not necessary to explain or to mention who made the action that is described on the sentence.

Here are some examples:

    1.  The stealers were arrested.

                 Obviously they were arrested by police officers.

     2.  The F1 car was fixed.

                 

                 It was fixed by an specialized F1 mechanic. 

     3.  The last TV that was at the store, was sold yesterday.

                 

                 It's obvious that it was sold by a salesperson.

     4.  The soccer game has to be stopped for ten minutes.

                 

                The only person who can stop a soccer game is a referee.

     5.  The class began five minutes earlier than other days,

                 

                Obviously the teacher was the person who started the class earlier.

When the agent is unknown.

The agent is used to be omitted when we don't have any idea of who can be the doer of the action.

The examples are the next following:

    

    1.  That car was parked infront of my house and I couldn't get mine out!

                

                Do you know who parked that car there?

     2.  My bike was broken, it didn't has brakes.

                 You have idea who broke it?

     3.  His book was scratched when he saw it on the table.

                 Do you know who scratched it?

     4.  The pen was lost and now it was left on my desk!

                 Do you have any idea who left it here?

     5.  The laptop was stolen on the classroom.

                 Do you know who stole it?