Important to remember is that 64-bit applications can only use 64-bit codecs/filters, and 32-bit applications can only use 32-bit codecs/filters.
The 64-bit versions of Windows can run both 32-bit and 64-bit applications. Most software is still 32-bit, so you will need 32-bit codecs for them. Only a few applications are 64-bit and thus need 64-bit codecs.
An example of a 64-bit application is Windows Media Center. Windows Explorer is also a 64-bit application. Both a 32-bit and a 64-bit version of Windows Media Player is included with x64 versions of Windows. The 32-bit version of WMP is used by default for compatibility reasons.
The codec pack contains both 32-bit and 64-bit codecs.