Why are parentheses used in quotes?
Usually to let the reader know that the quote was shortened in certain places.
If you want to leave out unimportant parts of a quote, you can insert (...) in those places. The reader will know that you left something out.
Sometimes parenteses are used to clarify a quote, as when a person is referred to as "he (=Malcolm)", but square brackets are preferable then.
Abbreviating quotes in this way may completely alter the effect it has on the reader. It is very often mis-used (abused).