They are easier to use then the Roman numerals. And the Roman number system did not contain a zero. This made it difficult for them to develop many mathematical concepts.
For the same reasons that we need a numeracy system today which is based on the Hindu-Arabic numeral system that was a lot easier than the Roman numeral system that it replaced.
Fibonacci introduced the Arabic number system to Europe in the thirteenth century (in the 1200s AD). At first the Europeans didn't take to the system, but later found it was much easier than using Roman numerals and the abacus.
SI uses base 10, just like our number system, so it is much easier to learn, remember and convert between units.
The Hindu-Arabic numeral system contains a 0 symbol which makes arithmetical operations a lot easier whereas the Roman numeral system does not have a zero symbol thus making arithmetical operations a lot more difficult.
The Hindu-Arabic numeral system (the numbers that we use today:0,1,2,3,4,5,6,7,8,9) replaced the Roman numeral system for calculating because it contained a 0 figure which made calculating a far lot easier than with Roman numerals.
Although the normal way to write the number four in Roman numerals is IV, that isn't much easier to write than IIII (if you count the pen strokes, it's 3 strokes vs. four strokes) and this system was intended by the Romans to be convenient to use, so they allowed that variation.
Arabic numerals are almost identical to our current number system and it is a lot easier than Roman counting as it gas base 10 not weird groupings of three or 5. It was just a logical step forward from the old ways
Because the Hindu-Arabic numeral system contains a zero symbol for positional place value purposes making calculations a a lot easier whereas the Roman numeral system does not have a zero symbol in it thus leading to confusion when calculating.
Yes, they are used as separators to make numbers more legible. For instance it is much easier to read 1,000,000,000 than 1000000000
cm --> mm is based on the metric system, which is in intervals of 10. This is easier than converting from who knows what from feet --> in. etc. (metric system is easier)
The new grid computing system at Advance America is much easier to install,manage and maintain than its old system since it was upgraded and made to be customer friendly.