Punctuation is the basic element of English grammar and without it a sentence is not only incomplete but also insensible. There are various marks of punctuation that are used in sentences to give them more meaning and make it easier for the reader to understand. If we omit these punctuation marks from a sentence we will be left hanging. Also, without a punctuation mark a sentence can have various meanings and the reader might assume the meaning the writer is not conveying.
Brackets are less common form of parenthesis and are used for special cases like inside of quotations or quoted materials. Brackets represent interruptions and there appearance signifies that the material inside them is added by someone other than the original author.
“Samantha took Ana [her sister] along with her to the party.”
Notice that the brackets  appear inside quoted text that are the direct words of someone. Also note that the text inside the bracket adds detail to the original text and is written or added by someone other than the real author.
In the above example, Lincoln’s famous sentence actually came mid-sentence and was not originally capitalized but the brackets were used to keep the integrity of the text and make the actual sentence grammatically correct.