1.We always round up numbers to make figures easy.
Eg. 10/3 = 3.3333. Rounding numbers to the nearest whole number 3 makes this number easy to understand.
2. Rounding numbers makes them simpler to use.
Eg. 3.33333. Is it easy to multiply 3.33333 x 5 or just 3 x 5. The latter is much easy compared to the former multiplication. We can easily write the ans as 15. But they are slightly less accurate.
3. For a considerably larger number, you need to round the number as they are comparitively much small.
Eg. Consider a large number 9,835,300.65. The value 0.65 is extremely small considered to the the neighbouring numbers (9,835,300). Here there is no need to worry about the accuracy as the value after decimal dot is comparitively small.