For some clients it is important that as little as possible is stored behind or under something else. If you've reached behind a coffee cup for a pen, you'll know this can be a recipe for frustration and disaster.
If you are right-handed it's good to have natural daylight coming over your left shoulder when you're writing. If, however, you're trying to see something on a computer monitor in bright daylight, you can be straining your eyes to read it. This problem will depend on the time of day, the time of year, and the position of your desk.