Читать книгу Collins Letter Writing - Collins Dictionaries - Страница 69

These are used when you need to add something to a sentence to clarify its meaning but which bears no direct relevance to the sentence itself. If you could adequately use commas instead, then use commas: they look neater and they indicate that you have a correct grasp of grammar.

Square brackets tend to indicate an omission in the text or something scientific. You should not normally need them in letter writing.