Grid is an important tool in graphic design as well as in digital design. The grid is a combination of imaginary lines that define the spacing and dimensions of the main layout and it's the foundation of an interface, on top of which the layout will be designed. It's a framework that gives the user the orientation and guidance to design every page and every content of a section, without having to worry about alignment, safe margins, and spacing. This is because the grid was already developed earlier to avoid these calculations for each page. Have a look at the following screenshot:

Photoshop guides with the Guide plugin

The grid is definitely essential on every design piece, but its importance is undeniable for flat. Since flat designs tend to have less boxes and lines, the grid is extremely important to hold all the visual elements together to form a layout. Two elements aligned can give the viewer the perception of a block or a line, and these visual perceptions are built...