I used to make ships and weapons and such, and use the graph paper as a figure of measurement if they were for a set or something. Or at least use the graph paper to draft some ideas for modeling into 3D. At other times, I used the graph apper as a kind of ground for other stuff that goes on that sometimes defy dimension. Kinda like the red spiders portions of XKCD and such. Heck, even making some battle scenes and platformer puzzles and game level designs and such was a purpose (great for flash animation or game design). It also helps greatly for roguelike game planning (DF Adventure Mode or DoomRL), tabletop planning for certain strategies or tactics, and etc.
I love graph paper for the amount of things you can do with it. It really helps with doing design stuff as well. Makes scaling a hell of alot easier, especially if using graph mode (tuned to the dimensions you are using on paper) in Illustrator or similar programs. A smart and lazy method is scan the image, and line up the graph of the image with the graph BG in the program to make conversion a breeze.
EDIT:
I also managed to get me a program that allows me to operate parts of my computer and/or mouse, and games that lack gamepad support, with my gamepad. Adapting the mouse to the gamepad has made operating the mouse a whole lot more precise. I'm not sure where I can put it to use, but I'm having fun with this program so far. Just got it adjusted to work for NO$GBA. Now to find a way to convert the DeSmuME saves and savestates over to this program.