A predicate is not a part of speech but a group of words that can have verbs, nouns, adjectives, and adverbs. These words work together to form what some call the verb phrase.
Although grammar does have rules, these rules help us communicate effectively. Grammar is the structure of a language, and the rules are the visible and obvious elements of that structure.
You can use passive sentences when you want to emphasize the action being taken, to avoid repetition, and when a situation applies to the general public.