Explain why passive rather than active verb forms are used in certain types of text

ANSWER: Passive verb forms are used to shift the focus off the agent and onto the subject. 

There are two nouns in a basic sentence; a subject and an agent. The agent acts on the subject. Here, the subject is bolded and the agent is italicized. 
I ate cereal. 

^ That is an active sentence, because it focuses on the 'I'. Below are examples of passive sentences. 

Cereal was eaten by me. 
Cereal was eaten. 

^As you can see, sometimes passive sentences don't even contain a subject. This is particularly useful when you want to divert attention from the agent, such as when your sibling asks where the last bit of cereal went (Doesn't 'The cereal was eaten' sound less guilty than 'I ate the cereal'?)

Hope this helped!