For the same reasons that we need to use maths today.
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Roman numerals are in general quite easy and straightforward to work out but for more complicated calculations the ancient Romans would use an abacus calculating device.
For addition, subtraction. division and multiplication purposes in the same way that we do maths today but they had their own system of numbers known as the Roman numeral system.
The same reason that people of other countries use maths.
good maths =)
Everywhere everyday you use maths for