SI means "Standard International." If you use units of measurement that aren't standard, you may end up with a meaningless result, or a result that no one else would understand. For example, if your watch ticks at 47 times per minute and you use your watch to time something, your result would be pretty useless to someone whose watch ticks at the (S.I.) 60 times per minute. Or if you measure something that ways 3 gromfs (I just made that up) to you, it would be useless to someone who uses kilograms. Therefore, we agree to use a specified set of units of measurement.
Some of the problems that may arise if Standard International (SI) units are not used include conflict or discrepancies in data collection since there is no standard measurement, conflicting results amongst the various research personalities as well as inaccurate representation. These are basically the derived units that are mostly expressed in terms of the base units.