There are situations in which Excel isn't powerful enough—for example, if you do a lot of research in a lab.
If you're doing heavy duty statistical analysis, Excel might not be powerful enough for your needs. For example, if you want to do regression analysis, Excel requires that the x values be in a single block (adjacent rows or columns), which might not be convenient for you.
In these circumstances you might want to use a dedicated statistical package, because it covers a larger set of statistical analysis options and related functions. Some packages also display additional outputs associated with a particular analysis. Typical users of these types of programs are people working in an academic institution's statistics department or workers in a research lab.
Many of these statistical analysis programs are commercially available. There are also specially designed add-ins for Excel from other companies. Some of them can be found in the Office Marketplace on Microsoft Office Online.