let's look at it this way...
we can access an array both ways, from the end as well as from the start ....accessing the array from the end you use -x
where x is any number that corresponds to the array element position that we wanna capture from the end.
so, look at this line now and tell me if 0 was not there will it not have been bizarre that we are interfering with laws of Algebra???
...you find that 0 is central to preserve and make intact your brains from having to dare to change the $[ or having to think where the central "Head" of the array is? since the "First in everything is the head" and a first element in an array is its head, and a first number in the one the makes both sides of the sign equated is the big round fat 0