Why there is a need of rounding off figures during calculation ?

The following rules are used to round off a number to the required number of significant figures:

a. If the digit following the last digit to be kept is less than five, the last digit is left unchanged. e.g. 46.32 rounded off to two significant figures is 46.

b. If the digit following the last digit to be kept is five or more, the last digit to be kept is increased by one. e.g. 52.87 rounded to three significant figures is 52.9.