When is the find() method actually necessary?
The ElementCriteraModel description in the docs explains what the find() method does.

Essentially, when you use craft.entries you are building an ElementCriteriaModel object that represents the query that you want to make. That query doesn't actually happen until you call the find() method.

As the docs state, as soon as you treat craft.entries (or a variable that you have assigned it to) as an array, by using it in a {% for %} loop, or by calling the | length filter on it, find() is automatically called.

So in the example in your question, you can safely leave out the find() and it will be called automatically anyway when the {% for %} loop is executed.