There are soooooooo many uses of rectangles (as well as any other geometrical shape, really).
I have a notebook right beside me, and in almost every single notebook around the world, the pages and front cover are in rectangular shape. Why couldn't it have been given a square shape? Or circular shape? Well, rectangular shaped paper actually gives you more space to write things down on both sides (as opposed to paper if it got a circular shape. Not only would there be enough space, but it wouldn't look professional). Not only that, but it can fit into backpacks and such much more easily.
I have an iPhone 5S, which is in rectangular shape (and seems to have stronger rectangular traits as Apple develops newer iPhones!). Why is that? Well, if we had a square-shaped phone, that wouldn't necessarily fit in our pockets as easily, and if it did, the screen would be too small (for a touchscreen, anyway)! As Apple develops iPhones that steadily increase in rectangular traits, it creates more room for the touchscreen, but still remains able to fit inside your pocket easily.
I know that these are just simple not-very-important examples, but in the real world rectangles are often used for the construction of buildings, and moreso used for applying earthquake safety and stability against earthquakes to lessen any damage that the quake might have caused. I don't know the exact specifics of where rectangles are used on a larger scale (other than this earthquake example), and you may not know it, but it's frequently used in daily life by a whole lot of people, either for big things, or somewhat insignificant things.