Clarification on one thing in this code.

You are passing in playList, you just choose to name it list within the printList function.

When you define a parameter for a function you are essentially telling JavaScript "take the argument passed to the function and assign it to a variable equal to this name". Since playList is the first argument passed in to it and list is the first parameter list ends up containing the contents of playList. The names of arguments and parameters does not have to match, JavaScript does the data binding entirely based on the position of the argument/parameter.

You could rename the parameter to playList like this:

var playList = [
  'I Did It My Way',
  'Respect',
  'Imagine',
  'Born to Run',
  'Louie Louie',
  'Maybellene'
];

function print(message) {
  document.write(message);
}

function printList (playList) {
  var listHTML = '<ol>';
  for (i = 0; i < playList.length; i += 1) {
    listHTML += '<li>' + playList[i] + '</li>';
  }
  listHTML += '</ol>';
  print(listHTML);  
}
printList(playList);

And it would work fine, but that arguably makes the code more confusing, because now at a glance it looks like the printList function is acting directly on the playList variable defined outside the function. Which is incorrect as it is actually working with whatever list was passed in, which could be any list not just the playList.

The printList is designed to take any list you want to send it, while it's implementation is a bit overkill when you have just one list in your program you generally should always design your function to be pretty generic so that they can be reused later in the program for other similar tasks. Like printing a different list to the screen later in the program for example. If you wrote the function in such a way that it manipulated the printList directly then you could not use the function with any other lists.