a) Variables are used to prevent the same computation being performed several
times as the variable allows you to store the values so that they can be used later.
b) A variable represents specific data or a value, and acts as a placeholder for that
value. Unlike a constant value, which is fixed and unchanging, a variable can be
repeatedly assigned different values. You assign a value to a variable and the variable
maintains the value until you later assign a new value. Because of this flexibility, it is
necessary for you to declare variables before you use them. Variables can be treated
as a normal attribute, that is, it can be a source or target of a transform.