A number is described as rational if it can be written as a fraction (one integer divided by another integer), e.g. 5/7. The decimal form of a rational number is either a terminating or a recurring decimal. A surd is an expression that includes a square root, cube root or other root symbol. Surds are used to write irrational numbers precisely. Because the decimals of irrational numbers do not terminate or recur, they cannot be written exactly in decimal form. To answer your question, you would use surds to better express irrational numbers, such as sqrt(3) and similar numbers, that cannot be expressed with ease. It might be more helpful if you give an example question.