Why is si system important in science?

The SI is a well-defined set of standards for measurement that can be checked for accuracy by various methods.

This means that all measurements can be compared with confidence: if I invent a new alloy with a particular strength, you can make that alloy and rely on the strength being the same as the value I stated.

Prior to the SI, there were several different sets of measures, and people needed to convert between them, such as converting from pounds weight to kilograms, or back the other way.

Mistakes in calculations could have dangerous outcomes, such as converting ounces to grams or grams to ounces for medicines. Get that wrong, and you would either have no effect (dose too low), or dangerous/deadly effects (dose too high).