That is, they should be made parallel. In a series, for example, nouns should be balanced with nouns, adjectives with adjectives, phrases with phrases etc.
Balance is the key word.
Without parallel structure, sentences sound off-balance and awkward. Look at the italicized section in the first part of the sentence. Look at the second italicized portion between the commas. Say aloud the three things that will be endangered. Awkward, it sounds awkward.