以下一句why要加上-ed

It is a common way to use passive voice, personally i think in formal writting,you still need to add "is" in your sentence,but there are another case that "ed" can be omitted even in formal writing:

Example: You should use the answer book provided.
Can you see that there are already one verb except "provided" --> should?
Actually,in this sentence,provided is not a verb,it is used as an adjective.Because in English grammar, there should not be more than one verb in one simple sentence.That's why an"ed" should be added to change provide from verb into adjective.

passive voice (被動式)