Graph paper, which is printed with a grid of squares, has many uses including mathematical graphing, art projects, and map-making in tabletop role-playing games. You can also use graph paper for note-taking to evenly space letters.
Graph paper is useful in any application where it is important that elements are evenly spaced. For example, making a line graph can be extremely difficult on unlined paper, since the spacing needs to be consistent to effectively display information. Graph paper simplifies this process, since one square in the grid can be equal to one unit of measurement.
For home improvement projects, you can use graph paper to sketch out a project to scale. If you are laying out a kitchen, for example, you can treat each square on a piece of graph paper as a 1-foot square, giving you a better sense of scale as you draw your renovation project.
You can also use graph paper to create pixel art, treating each square as a single pixel of a larger drawing. Another use for graph paper is laying out a website, since you can treat each square as a pixel and figure out how page elements will look in relation to one another.