When a calculator makes a calculation and displays it on a screen, it can only display the numerical answer to a given length of decimal places (usually 8 or 9 for calculators), so it needs to round the final digit of a repeating decimal, in order to display the answer.

In computer programming, you may also need to make a calculation resulting in a decimal, but declare the result as an integer value, so a subroutine will round the decimal to an integer.

There's probably many more obvious applications but I can't actually think of too many. Maybe like measuring instruments are only accurate to a certain degree like for a ruler, could be another.