While it is very easy to dump a CSV text file from your C ++ program, it is not user friendly or aesthetically pleasing, and even worse, there is a risk that malformed data may corrupt the file. Exporting directly to Excel provides a better experience for your users, by automating some of the manual tasks your users would have to preform if they were to import the CSV into Excel themselves.
As CSV files are unstructured text, so they are difficult for users to read in their raw state, so they are almost always imported into Excel for this reason, forcing the user to do unnecessary work. Even then, the data will not have any formatting which will make it more difficult to understand than it should be. When exporting the same data to Excel with a C ++ library, you now have the ability to add formatting and titles to make it more readable, resize columns to better suit the data and add tabs to more logically lay out the information.
In addition, an Excel file exported from C ++ also allows you to add formulas and charts that greatly increase understanding, but would have to be manually added by a user when exporting plain text files. Pictures can also be embedded directly into the single exported file to improve aesthetics, which is another improvement over CSV files.
If the output data is not carefully processed and escaped, it may corrupt the CSV file. And in the worst case, if a developer is not available to fix the exporter, the user will be required to hand edit the CSV file every time it is exported to fix the corruption. Using a good C ++ library for exporting Excel files will avoid this problem entirely.
Also, a well designed programming library for Exporting excel files will actually be easier to use and support in the long term than CSV code, as it allows for more understandable code to be written. Code to export CSV files typically devolves into one mega line of code that concatenates all your strings and variables together and writes them directly to a file, compare this to an excel library that allows you to access the cells directly instead, allowing you to split up the writing task per value or reorder as you see fit.
These differences make Excel the better choice over CSV for exporting data from your C ++ program, saving your users time and pain in the long run.