Excluding mathematical uses, in American English the primary use of the bracket [] is to show when text has been changed in a quote. For example:
I don’t drink water. Fish [swim] in it. – W. C. Fields
Using brackets like this is usually done to clarify the meaning, or to shorten a quotation.
https://thebettereditor.wordpress.com/2012/01/16/a-brace-of-brackets/