Including or omitting the indefinite article is entirely a matter of personal style. In some cases it sounds better to include it, to help emphasize the individual elements of a list.
In the picnic basket he carefully packed an apple, a pear, a bottle of wine, a round of cheese, a dish of pâté, a pat of butter, and a freshly-baked baguette, as well as a corkscrew, a sharp knife (for the fruit and the cheese), a serrated knife (for the bread), a blunt knife (for the pâté and the butter), a small china plate, a silver fork, and a single, long-stemmed wine glass.
Note how the repetitive article makes it clear there is one of each object. It also helps reinforce the impression that the subject is a meticulous person who has carefully chosen each of the items in the basket.
Remove the repetitive article, and the sentence sounds more hurried and careless:
In the basket he tossed an apple, pear, bottle of wine, round of cheese, dish of pâté, pat of butter, and a freshly-baked baguette.
There are many other reasons why a writer may choose to repeat the article, but the best way to learn these is to read copiously.