Apple recently announced that they are no longer supporting QuickTime for Windows and most importantly, will not be providing security updates going forwards.
We use QuickTime in the viewer to play back media type likes MP3 MPEG-4 and of course MOV.
This viewer replaces the QuickTime media plugin for the Windows version of the viewer with one based on LibVLC (https://wiki.videolan.org/LibVLC/).
The OS X viewer is unchanged since Apple will continue to support QuickTime there.
Eventually, as part of the update to 64 bit versions of the viewers, the OS X viewer will also be updated to use LibVLC (QuickTime APIs are Carbon and not available as 64bit).