js reduce method point of array

In the reduce method the following works:

let dolphins = [92,108,89]
dolphins = dolphins.reduce(function(a,b){
   return a + b/dolphins.length},0)

note that inside the reduce function we have access to dolphins.

From the documentation we have:

array.reduce(function(total, currentValue, currentIndex, arr), initialValue)

Parameters

Parameter   Description
function()  Required.
A function to be run for each element in the array.
Reducer function parameters:
total           Required.
                The initialValue, or the previously returned value of the function.
currentValue    Required.
                The value of the current element.
currentIndex    Optional.
                The index of the current element.
arr             Optional.
                The array the current element belongs to.

What's the point of passing the array since the function already has access to it?

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