As Roland mentioned, most R functions need it in long format, and it is often easier to process data that way.
But on the other hand, it is easier for people to view and comprehend wide format, especially when it is being input and validated, where human comprehension is important for ensuring quality and accuracy.
So I see that data tends to start out life in wide format, and then become long as it becomes used more for processing. Fortunately converting back and forth is pretty easy nowadays, especially with the tidyr package.
