Why scientific notation used?

Answer 1

Mainly, they are used in science where numbers are way too large or way too small to write out.

Example:

Instead of 84782384756293847462938475923938

We would write 8.478 x 1031

or

Instead of 0.00000000000000000000000347

We would write 3.47 x 10-24

Answer 2

To compute very large or very small numbers.