The primary reason for writing Microsoft Office Excel 2007 XLLs and using the C API is to create high-performance worksheet functions. The applications of high-performance functions—and, in Excel 2007, the ability to write multithreaded interfaces to powerful server resources—make it a very important part of Excel extensibility. The performance of XLLs is further enhanced in Excel 2007 by the addition of new data types and, most important, support for multithreading.

The C API has none of the higher-level rapid development features of Microsoft Visual Basic for Applications (VBA), COM, or the Microsoft .NET Framework. Memory management is low level, and therefore puts greater responsibility on the developer. Many Excel features that are exposed through COM, making them available through VBA and the .NET Framework, are not exposed to the C API.

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