The next major Windows 10 release will reserve 7 GB of your device's storage space to avoid failing big updates due to a Windows Update bug. Due to this bug, the system is not checking for enough space before launching after an update. While 7 GB might not be much for most Windows PCs, cheap devices with 32 GB of internal storage will get in trouble.
https://www.notebookcheck.net/The-next-Windows-10-version-will-feature-reserved-storage-and-you-might-not-like-it.392971.0.html