For what purpose do you need to know the Arithmetic Operators?

Answer 1

to do almost all basic calculations inside program for example a loop
for(var i=0; i<9; i++){
sum+=0;
}

Answer 2

Computers were programmed to think with those operators. They also learned it as PEMDAS. It will look for parentheses first in your code, then exponents(or functions that do the same), then it will multiply or divide, then add/subtract.
Knowing how those work will help you in coding and debugging that code.