Question

Why are SI units helpful?

Answer 1

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.

The Mars Climate Orbiter mission provides a great example of why using SI units is so important.
http://en.wikipedia.org/wiki/Mars_Climate_Orbiter