In my opinion it's like a one language, for example when you are trying to work with collaboration, and let's assume that the people around you are speaking in different language, it makes no sense if you all start to figure out what's going on because you can't understand each other... But if everyone speaks in one language, everything will work like a charm.
In this case scientist uses international system of units because, if he/she needs some help from others, he/she can easily ask others because everyone can understand those units that he/she is using....
I hope it makes sense.
