Often when a painter is faced with a scene, there's simply so much that's appealing it's hard to choose what to focus on. This is where a viewfinder comes in useful, as it helps you focus on particular parts of the scene, enabling you to decide what will make the best composition, both in terms of focus and format.
If you look at the landscape in the photo above (view a larger version), you'll quickly see the potential for various landscape paintings. By using a viewfinder to isolate just a part of the landscape and to frame it in a particular way, it's easier to judge whether a composition is pleasing or not.
