In both cases the artists have created a record which could be used on its own or to complement a photograph. Salient features which capture the mood of the scene have been recorded (the particular way the shadows fell across the mountains, the banding in the clouds out to sea). Being in colour, the sketches also serve as a useful aid for comparing your memory or perception of the scene with a photo. Although there has been no attempt to record the scene in detail here, you can use sketches to record something particular which may form a focus in a painting. And lastly, it allows an atmospheric record of the light and weather, which are sometimes lost in a photo.
Sketching is ultimately just part of the process of creating art works. Whether you set out to record particular aspects of a scene, or allow your brush to develop the record as you go (connecting with a more subconscious level of your creativity), you should be able to re-discover what inspired you about the scene in the first place when you're working back in your studio.
