When may it be a good idea to use scientific notation of si prefixes?

It is a good idea to use scientific notation to express very small numbers or very large numbers.

Explanation:
Suppose you carry out a calculation, and the final result requires diving 3 by 5 million.
The answer would be  written as 3/5,000,000 = 0.0000006.

Instead of writing six zeros before 6, scientific notation expresses the answer as 6.0 x 10⁻⁷ or as 0.6 x10⁻⁶. This means that the decimal before 0.6 should be moved left, with six zeros preceding 6.

Similarly, multiplying 3 by  5 million yields 15,000,000. Instead of writing all those zeros after 15, scientific notation expresses the result as 15 x 10⁶ or as 1.5 x 10⁷.

SI units provide several prefixes for expressing small and large numbers.
For example, the prefix milli means 10⁻³ or one thousandths, such as a millimeter. The prefix kilo means 10³ or one thousand, such as a kg (kilogram).