There are many reasons, but the two that strike me most strongly are given below:
They allow us to make consistent measurements over time (I can measure
They allow us to set international standards (safety, EN standards/ISO standards) because they are consistent across space.
I’ll be interested to see other reasons pop-up from different contributors so I’ll request a double-check and hope others do too. More heads = better learning!
Standardized units of measurement can help avoid errors from mis-understanding.
Such a mis-understanding doomed a shot to Mars as was reported in this article:
Apparently data was exchanged without including the units used (or without noticing that it was not in the expected units). (It involved data calculated in pound-seconds being interpreted as being in Newton-seconds. The LA-Times article called the data force data, but looks like impulse data to me.)
Perhaps the use of the English system should be banned from science.
I hope this helps,