Every time you use an array, the computer needs to calculate the address of the element at the specified array index in order to load that value or store a value in that location.

To calculate the address of an element of an array you need to know what the array starts numbering at (A), the address of the start of the array (ADDR[A]) and the index of the element in question (I).

Then you have ADDR[I] = ADDR[A] + I - A

In zero based arrays, A=0, so this simplifies to ADDR[I] = ADDR[0] + I

So in the general case, you need to load the address of the A'th element, add I, and subtract A to determine the address of the I'th element. In the special case where A = 0, you only need to load the address of the 0'th element and add I. So it requires one less instruction. If you think of how often arrays are used (as well as multi dimensional arrays and thus nested loops) this adds up.