When will we use rounding in our lives?

Answer 1

Whenever we use numbers for which the detailed digits, after a certain number of significant figures, are not known or do not matter in terms of communicating the size. Pi is irrational and so has an infinite decimal representation but most people will use the value of pi to at most 6 sig figs (the diameter of a hydrogen atom at the furthest point of the known universe will subtend an angle for which 39 digits of pi are sufficient).

There are many socio-economic measures which can be known only in rounded form - either because of uncertainty of measurement or the fact that the numbers are constantly changing. The world's population is not known to the last integer - by the time you record the number it has changed; the GDP of a country can only be estimated to an accuracy of hundreds of thousands of currency units (measurement errors); and so on.