The passive voice is generally used when the subject of the sentence is indefinite, general, or unimportant. In the sentence, They mine coal in Pennsylvania, the subject is so indefinite that it is not clear what is meant by they. It might mean the miners, the people, or the companies. This sentence, and sentences like it, can be improved by putting the verb in the active voice.

They mine coal in Pennsylvania. (poor)
Coal is mined in Pennsylvania. (better)

They use tractors on some farms. (poor)
Tractors are used on some farm. (better)

The passive voice is also used when what was done is more important than the doer of the action. Look at the following sentence:

America was discovered by Columbus. (passive)
Columbus discovered America. (active)

The use of the passive voice is generally used when you want to emphasize the receiver rather than the doer. However, in the great majority of cases the active voice is more effective than the passive voice.