Grids, in graphic design, are very important. They are so important that one of my good friends actually did her Master's thesis on the subject of grids.
But what is a grid?
Basically grid is a two-dimensional structure made up of a series of intersecting vertical and horizontal axes used to structure content. The grid serves as an armature on which a designer can organize text and images in a rational, easy to absorb manner that is pleasing to the eye. So this makes them one of the essential tools in design.
Grid can help you easily design a poster, a brochure or a web page. And even when it doesn't seem like there is a grid by which something was designer, you better believe there is.
Once you find out how grids can help you, that's when you'll also realize how they can make your life miserable. There is nothing more frustrating than wasting you precious project time on something like creating a grid, especially in Photoshop, and no matter how skilled you are, you will never be able to hit that exact pixel you were set on. Or if you're distributing guides by "New guide" command you can get your calculations wrong, and who wants to be distracted by diving space in columns and rows with perfect blank space. Not me for sure!
But Google is your friend and sometimes it is easier to google a solution for your problem, than actually solving it by yourself. It is a typical designer's disease that goes by the name LAZINESS. There is no cure for this thing, if there was I'd be the first one to stand in line.
Until the cure is found, I will be satisfied with a tool named "GuideGuide".
photo taken from http://guideguide.me/