Personally, I don't like the grid method for use in digital arts and especially if your goal is to learn. I feel like the main point of this exercise is to show you a good tool when drawing from life. While drawing the grid on top of the reference helps copying the reference, there's not much benefit over simply tracing it.
However, if you wanted to make an accurate copy from a photo in a traditional medium, the grid would be the way to go.
The main reason I don't like using a grid is because it takes away a lot of the observational skill required while drawing.
All in all I would say this lesson is more of a "nice to know" and less something that you'd practice daily anyways.