Arrays in JavaScript (and many other languages) are 0 indexed. So the index of the 4th element of the array arr is 3 which is also a multidimensional array.
For me, flattening it out a bit helps me see it better…
I don’t think there is any particular reason. The example is showing you how to access multi-dimensional arrays and used the fourth item in arr probably because it is the most interesting since it has two levels of sub arrays.
It’s kind of like one person lives at 123 Mainstreet…
another person lives at 456 Grand Apartment 5…
and yet another person lives at 678 2nd Ave Building 8, Unit 9…
indexing is kind of a way of addressing where the element is in the array, and so you have to go deeper into a multidimensional array to access each element within it.