Why Do They Put Brackets In Quotes & Sayings

Especially with girls, people always want to put them in little brackets. — Gemma Arterton

He put down the receiver and looked vaguely at the paper in his hand. It was a rough piece of white wrapping paper. Scrawled in pencil in ragged block letters were the words:
HE DISAGREED WITH SOMETHING THAT ATE HIM
And underneath in brackets:
(P.S. WE HAVE PLENTY MORE JOKES AS GOOD AS THIS) — Ian Fleming

(It's a weird thing, depression. Even now, writing this with a good distance of fourteen years from my lowest point, I haven't fully escaped. You get over it, but at the same time you never get over it. It comes back in flashes, when you are tired or anxious or have been eating the wrong stuff, and catches you off guard. I woke up with it a few days ago, in fact. I felt its dark wisps around my head, that ominous life-is-fear feeling. But then, after a morning with the best five- and six-year-olds in the world, it subsided. it is now an aside. Something to put brackets around. Life lesson: the way out is never through yourself.) — Matt Haig

A few words which he wanted to emphasize were put into brackets or set off by quotation marks. My first impulse was to point out to him that it was ridiculous to put slang words and expressions between quotation marks, for that prevents them from entering the language. But I decided not to. When I received his letters, his parentheses made me shudder. At first, it was a shudder of slight shame, disagreeable. Later (and now, when I reread them) the shudder was the same, but I know, by some indefinable, imperceptible change, that it is a shudder of love- it is both poignant and delightful, perhaps because of the memory of the word shame that accompanied it in the beginning. Those parentheses and quotation marks are the flaw on the hip, the beauty mark on the thigh whereby my friend showed that he was himself, irreplaceable, and that he was wounded. — Jean Genet