For example, when we see the advertisement 'TOMATOES ARE SOLD HERE', do we really care the name of the seller or the farmer? Not at all. The important thing is the fact that you can buy tomatoes here. On the contrary, if Brad Pitt was the person selling tomatoes, we would never use the passive voice and we would say: Do you know what? Brad Pitt sells tomatoes on the road to Granada.- You only include the agent when it gives some information.