Why are there brackets around some words in articles? Why, in articles, are there brackets around certain words, when the sentence would otherwise make no sense?
To make it fit to your writing.
Often times, writers take quotes that aren't complete sentences, and even more often, those sections don't really fit the way the writer wants them to. So he'll add a few more words or maybe change some of them (usually tenses or helping verbs) to make it fit his writing. When he does this, he indicates the added/modified sections of text by placing them in brackets.