Nothing. This article is demonstrating a proof of concept using a library without having to install Excel on a server. If all you want is a straight dump of data from SQL Server to Excel then the Excel Destination is fine.

If you want a more complex spreadsheet produced then the library will allow a much more tailored output. Apache POI allows something similar and there is a .NET port called NPOI on codeplex. http://npoi.codeplex.com/

Where it really gets interesting is that these libraries allow you to read Excel and all the bizarre formatting that gets put into a typical workbook.

I'm sure there will be an excellent follow up article on doing just that.

This is a good introduction starting with a nice simple example. The author hasn't fallen into the trap of producing an overly complicated example.