While fractions are more exact, it will always depend on the scenario. You aren't going to give the amount of money something is as a fraction, that will always be a decimal. However, in most math circumstances, fractions can get more straight to the point since there is no such thing as a truly repeating fraction.
It is because the formula for the circumference of a circle is 2 x 3.14 x r so I do the 2 x 3.14 and I get the 6.28 and then 6.28 x 3.5(radius) is 21.98.