Why is a single system measurements necessary throughout the scientific community

It is necessary because using different measures can lead to various issues that can even be deadly sometimes. An example can be if i tell you that something needs to be prepared to a 0 degrees, but i don't say that it should be 0 degrees celsius, you might get it to a zero degrees kelvin which is - 273 celsius, a temperature where atoms stop moving completely. Or you might mix chemicals improperly and cause explosions.