Brackets are used to mark off from a main sentence a group of words which add an explanation or make a correction.

His look seemed weighted down by the long years in which he had served (and saved) his country.

When winter’s evenings fall early (sitting at her spinning wheel) she bids defiance to the giddy wheel of fortune.

Everyone taking a sherd (a piece of earthenware) wrote upon it the name of the citizen he wished to be banished.