Other than the physical use of brackets to hold up bookshelves, most fiction writers don't use brackets very much. But their traditional use is to enclose words and phrases that are independent of a sentence. This can include explanatory notes, omissions, and comments. For instance:

Note that no punctuation is needed with brackets, unless they contain an entire sentence.

For “aside” comments and other phrases that don't belong in the sentence itself, it's best to use parentheses:

Don't use a comma, semicolon, or colon in front of the opening parenthesis (the singular form of the upright punctuation). You can use a comma, semicolon, colon, or period after the last parenthesis.

By including subject matter in this way, the author calls attention to what is inside the parentheses. Of course, commas or long dashes may also be used to set off parenthetical text.