The use of focus or blur in photography is a native characteristic of the medium that can be used to great effect. Focusing on subjects draws attention and importance to them. Blur, or showing subjects out of focus starts to bring a layer of abstraction to them. The amount of blur is at the discretion of the photographer – a little bit of blur can create a sense of abstraction, nostalgia or even subconscious representation. Extreme blur can turn shapes into simply textures. Careful use of focus and/or depth of field can create a great amount of interest in what your are communicating visually.
Here are some photos I've taken:
On the left photo, the focus is on the piano and it shows the depth of the image.
On the photo on the right, the main focus is on the top part of the hair dryer and the bottom part is slightly blurred, which draws the attention to the top of the hair dryer.