When you are working with a software that provides a proprietary or application-specific data format for storage, it can be useful to use this format. The SAS7BDAT format has a lot of advantages over CSV if you are working within SAS because it can be read and written quickly and preserves column type information, among other benefits. When you are using .rdata or .rds files with R, you can write and read objects from your R workspace with a single function call. Even complex objects like models, which are usually represented as complex, nested lists with elements of various different types, can be stored and retrieved just as easily as a single variable.