Windows Media Player and DirectShow
Windows Media Player can use installed DirectShow filters and codecs. Adding those components may allow it to open more audio/video streams without replacing the player.
Default apps are separate
Installing codecs does not automatically make Windows Media Player the default player. File associations are managed in Windows Settings and are covered in the site guides.
Useful playback examples
The pack may help with MKV, MP4, AVI, MOV, AC3, DTS, FLAC, XviD, DivX, H.264, HEVC and other common playback combinations in compatible software.
When the player is not the issue
If the same file fails in several players, the file may be incomplete, damaged, encrypted or encoded in a way that needs conversion or repair instead of a codec change.
Practical takeaway
Use the codec pack when a DirectShow-based Windows 10 player needs extra decoding, splitting or filtering support. If the file is broken, encrypted or outside normal playback profiles, a repair, conversion or different player may still be required.