Designers use a grid as a tool/system to help organize space. Instead of having random things flailing around in their design, designers employ a grid to help impart a sense of unity or to aid them in controlling initial layout. Grids are especially useful because they are so flexible; just about any design can use a grid. Because there is no set limit number of columns and flowlines, the possibilities are endless.
On another note -
I found this nifty website when I was doing tonight's homework. I have to say, the "problem/solution" layout was appreciated. It was also easier to understand than some of the other websites I visited. I liked that the website provided some logic behind the solutions it gave. None of this "it's right because I say it is" crap.
Everything you always wanted to know, but were too afraid to ask...
"sznurowka" = shoelace in Polish