Check to remove unnecessary old updates. Troubleshooting: Common Issues and Fixes
It holds the installation files for optional Windows features, such as .NET Framework 3.5 , which can be enabled without needing the original installation media. Managing Disk Space: How to Safely Clean WinSxS
It keeps multiple versions of the same system components, allowing different apps to access the specific version they need without conflict.
If you cannot enable .NET 3.5 through "Turn Windows features on or off," you can use the DISM tool to point directly to the sources\sxs folder on your Windows 8 installation media.