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

I believe SI is the international system of measurements, our metric system. You would want to use this when numbers begin to get too small, or too large. Some examples would be when calculating distances of spacial bodies such as the distance from earth to the center of the milky way which is about 1,000,000,000,000,000,000 km or 1*10^18. 1*10^18 is easier to write and is less confusing.