Why does it have to say (numbers[index])

Your basically try to access the value at a certain index in the array. Its like if you have a cargo in a cargo box, the cargo box is identified by a number, to get the contents you have to have the cargo box number. So its the same thing here, to get the value in the array, you need to access it using its index, and this index is in fact a pointer which points to a memory address of the value. And then to get the next index, you have to increment your current index by one, so the pointer will move to the next address. Hope this helped.

It might also help to see what's happening behind the scenes at each step as the program goes through the while loop in your code:

// Step #     Index is     What the print function looks like on this step     Output
// 1          0            println(numbers[0])                                 1
// 2          1            println(numbers[1])                                 2
// 3          2            println(numbers[2])                                 3
// 4          3            println(numbers[3])                                 4
// all the way to...
// 10         9            println(numbers[9])                                 10
// and we exit the while loop. (because index becomes 10 on the next step and equals numbers.count)

Index is a variable to take the place of 0, 1, 2, etc. (the array indexes) so we can use a loop and add 1 to it each time rather than write out 10 println statements. Image how hard that would be if you had 100 numbers instead of just 10... Loops make our work so much easier!