Why is a universal system of measurement necessary in science?
The scientific community needs to communicate.
A universal system reduces the confusion when different systems of measurement are used and makes it easy to compare measurements taken by different persons.
Here’s a real-world example of the confusion that can occur.
In 1983 an Air Canada Boeing 767 temporarily had no working fuel gauges, so the ground crew resorted to calculating the 767's fuel load by hand. They used a procedure similar to calculating the volume of oil in a car by taking a dipstick reading. This gave them the volume. But airlines measure the amount of fuel by mass. They needed the density of jet fuel to make the proper calculations.
The ground crew used 1.77 lb/L as the density, as they had on all other planes in the fleet. But the brand new 767 was metric and used 0.8 kg/L as the density. The plane had only about half the fuel the crew believed they had. The plane ran out of fuel and sank too fast to reach its destination. Only a nearby abandoned Air Force base was a possible landing spot. The crew made a remarkable dead-stick glider landing. There were only minor injuries to the passengers as they used the emergency exits on the plane.
You can read this remarkable story at http://www.wadenelson.com/gimli.html