They are incorrectly used as a pause in the writing, and students are sometimes taught to insert commas when the reader should breathe when reading. There are basic rules which dictate the placement of commas. If the items you are listing have a roughly equal value in the sentence, they should be set off with commas. See the section on semicolons for information on punctuating complex lists. Adjectival phrases can be confusing if punctuated improperly.
Noncoordinate adjectives are ones which modify one another.
Sometimes a sentence can be interrupted by a word or word group which adds to the intended meaning or relates the sentence to its surroundings. Notice the difference in the implied meaning of the following sentences. The swimmers already in the water were exposed to a large number of parasites. The swimmers, already in the water, were exposed to a large number of parasites. Example 2 assumes that there is only one group of swimmers and they happened to already be in the water.