They are used less frequently than parentheses and are most often used in relation to quotations.
Use brackets to enclose words that you have added or changed in a quotation.
This doesn’t mean that you don’t use any other punctuation after that period.
Use all of the regular punctuation that you would if that “etc.” were just another word – question marks, exclamation points, semi-solons or colons.
In that case, use whichever works best for your situation.
You probably know it better as “etc.” you have read things that have used it; you have probably said it yourself.
Square brackets are also commonly used by writers who feel the need to italicize certain words for emphasis: Researchers at the Institute of Technology walked out of the presentation, saying they were [emphasis added] disturbed by the images.
The brackets above indicate to the reader that the italics that are emphasizing “extremely” have been added by the writer, and not the original author.
Generally, braces, brackets, and parenthesis are used to set off additional or optional information in sentences.
The writer may need to add words to clarify meaning, make the sentence read more clearly, or make a comment or correction to quoted material.