While parentheses can permit you to write more complicated regular expressions, their main purpose may be to let you extract substrings from a matching string.
All numbers have two digits to the right of the decimal point.
There may, or may not, be any digits to the left of the decimal point. Here is a pattern to match those numbers. Now, suppose you want to search for a parenthesized negative number, extract the nonnegative number in the parentheses, and make it negative. They are assigned substrings determined by the way you add parentheses to your pattern. And so on.