why is it important to insert parentheses when multiplying expressions in which numerators and the denominator?

Because the order of operations (also called operator precedence) dictates that x + 2 / x - 3 be interpreted as x + (2/x) - 3, unless you put parentheses in there.

If you're entering them into a calculator, the parentheses are the only way the calculator can sort out the order of operations correctly. (except if you are using Reverse Polish Notation, which is sadly out of fashion these days)

If you're posting your expressions somewhere like yahoo! answers, there's no way to format them properly as fractions with horizontal lines below the numerator and above the denominator, so the parentheses are an immense help to those of us trying to answer your question.

Consider what happens when you do not include them!

e.g. 1/3x^2 - 4/7y^3 can be (1/3)x^2 - (4/7)y^3 OR 1/(3x^2) - 4/(7y^3) ~~ see the difference?