64-bit

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.

The 64-bit codecs have better performance. For common formats such as H.264 video, the difference is relatively small, about 10%. For newer formats such as HEVC and VP9 video the difference is much larger, up to 100%. So it can be beneficial to use a 64-bit player.

One reason to use a 32-bit player is compatibility with legacy codecs for which no 64-bit version is available. The codec pack contains 64-bit versions of all important codecs, so this only matters if you have any old third party codecs installed. You would known if you have any of those.

Another reason to use a 32-bit player is if you want to use madVR, because that is currently only available as a 32-bit version. madVR is an advanced video renderer for high quality video playback.