Talented artists who can render accurately have amazing EYES. It's not just the hands that draw. We would even argue that hands aren't even the most important tool in drawing. Drawing begins with the eyes and an awareness of how these angles and shapes fit together.
Grids break up the large page into smaller, more-manageable spaces. They give us a roadmap to follow. It's easier to plot a point and hit the target if we narrow the playing field!
Our artists found it easier to work in one square at a time and to "jump" around, instead of working in neighboring squares.
We followed a 1:1 ratio on our grid drawings so that we could check our accuracy after we were done by overlay our drawing over the original.
We used simple line drawings, but this exercise works with photographs as well.
(Easter-themed, because we're dreaming of spring!)