It's called Non-Finito and It's a stylistic choice. It may imply to the viewer that the artist painted this quick, like He was overcome by the idea and just wanted to put it on paper before the idea went away.

It's also an intriguing because it engages the Imagination of the viewer. They try to complete it in their mind or try to imagine what might fill the empty areas. This can be seen as a way to Connect the artist and the viewer.