We rationalize the denominator to ensure that it becomes easier to perform any calculation on the rational number. When we rationalize the denominator in a fraction, then we are eliminating any radical expressions such as square roots and cube roots from the denominator.

Why is it useful to Rationalise the denominator?

Rationalizing the denominator (RTD) (a special case of the method of simpler multiples) is useful because it often serves to simplify problems, e.g. by transforming an irrational denominator (or divisor) into a simpler rational one. This can lead to all sorts of simplifications, e.g. below.

Do you always have to rationalize a denominator?

Explanation: Technically no. The general reason why it is desirable, is to have a standard form. If for example you look a trig ratios that have radicals, these are given with rationalized denominators, so it makes it easier to recognize these ratios when you rationalize the denominator in your calculations.