SI units provide a way for scientists around the world to record information about observations in a manner that all scientists around the world will understand.
Imagine if scientists in the US had one way of measuring mass, scientists in Europe had a different method, and scientists in Asia had a third method. Nobody would really know what anyone else was talking about.
SI units are easy to convert because you multiply or divide by 10 - sometimes more than once. Think of a staircase - every time you step up a stair, you divide by 10; every time you step down a stair, you multiply by 10.
Another way to think of it is: as you move down the staircase, the decimal moves to the right; as you climb up the staircase, the decimal moves to the left.
If you need to memorize the order of the prefixes, there are a number of mnemonics that can help you. My personal favorite is "King Henry Dropped over Dead Converting Metrics."