A lot of designers are constantly fighting the the urge to "fill the space". When composing a layout, poster etc. We try to use whitespace to help improve the composition to communicate as best as possible.
I look at whitespace like I look at a good type family. When they are used, a great type does not sit on top of the plane but it sits inside of the plane. Not sitting over or covering the space but inhabiting the space and sharing it with the emptiness.
I scale that up to composition elements when applicable. When designing a layout I try to make sure the headers, body text etc simply do not just take up space over the plane. But these elements sit inside of the plane themselves, creating a relationship between elements and empty space.
These empty spaces are vital to the compositional flow of any look and learning how to use space to direct the eye is super important to any successful solution. Some designers and even clients get aggravated with too much or little white space because "There is to much blank space, you hardly did anything etc." Now maybe they are right...that is another issue altogether. But let us just say they are wrong. Whitespace that is used correctly can be hard to come by and makes a design stronger. So to get to the best solution which looks like minimum work sometimes requires massive amounts of work to get there. Simple does not always mean easy.
Taking time to learn how to dictate a viewers eye and command a composition is going to pay great dividends in the long run. Learn when a design is too busy or maybe when there is too much space. The constant battle of balance is what we are left with.
"Less is not always more, just enough is more." Milton Glaser.