Why is rounding helpful?

Answer 1

It is because in most instance people are do nor need nor are they interested in the detailed number.

The number pi has been calculated to over a trillion digits but with just 40 digits, you can measure an angle subtended by a hydrogen atom at the far end of the known universe! The additional digits are fun for mathematicians but are of no practical use.

If asked for my height I will say 1.78 metres. I doubt if anyone is interested in the number at the third decimal place (millimetres) or at greater detail. Moreover, my height is not constant: over the course of a day I shrink by approx 1.5 cm as my spine settles but regain that height when I sleep. So the accuracy of any measurement given to more than 2 decimal places (in metres) is spurious.

There are also other practical applications. Unit prices are usually rounded to 2 decimal places because that is the normal relationship between the major and minor currency unit. So 2 cans for 75 cents is 37 or 38 cents per can - never 37.5 cents.