In order to provide modulo (%) facilities as far as mathematics and logic is main concern. for instance, if we have no. of employees in our list say 1545 and if we want to accommodate it within small and effective range then by using % we can do it easily. we can arrange it in the range within Array with maximum index 14 (0,1,2,.....14). And 0,15,30,45......1545 will belongs to same category say index no 0. Think if 0 is not included into index then where we would store 0,15,30,45....1530,1545 . From a technical standpoint, you need to understand that an array is basically just the starting address of a chunk of memory. The "index" of the array is actually an address offset. So the value at the 0th position would be stored at the address of the array plus zero. The value at the 1st position would be stored at the address plus one, and so on.
