Windows Central has an article today pointing out a Microsoft blog post from last month -- it finally gives some more info on the way Windows 10 can compress itself... previously Microsoft would say basically what it did, & they gave details on using it with tools in their automated installation kit, but that was pretty much it. The purpose of Compact is to free up disk space, primarily on mobile or other devices without a lot of storage capacity, that use SSDs rather than conventional hard drives -- if you use it with a regular hard drive the odds are higher that you might notice a performance hit.
Another use would be if you're running 10 installed to a USB or VHD drive where reducing the installed footprint is more important than speed, e.g. on a 16GB USB stick. [Having a [Insider?] copy of 10 installed on a USB stick with say your backup software installed for example, along with any other tools like file recovery apps might be handy. Installing 10 to a VHD can be useful for another set of reasons -- in many ways it's like a VM (Virtual Machine) without many of the hassles & liabilities (like fewer hardware resources available).]
Well, it turns out that Compact.exe is a command line tool or program with several options you can use whenever you want or need to. You can let it run automatically & it'll compress a predefined list of Windows files in a "single hidden container (basically a WIM file)" [per the Microsoft blog]. In Windows Explorer the Windows folder properties show the same number of files, & the same total size, but the reported on-disk size for the entire partition is about 2 GB smaller -- after running it on a bare Windows 10 Pro 64 VM, it reported that it had achieved a 1.7 - 1 compression ratio.
To run Compact in Windows 10 you need to start with a command prompt window running as admin -- to get that just right click on the Start button. Type "compact /?" [w/out quotes] & press Enter, & you'll get a list of all the available options. "COMPACT.EXE /CompactOS:always" turns it on -- "COMPACT.EXE /CompactOS:never" turns it off [both w/out quotes]. Instead of always type query to find out if it's on or off. Other options let you compact specified folder trees, with/without including .exe files etc.
windowscentral.com/how-reduce-windows-10-footprint
msdn.microsoft.com/en-us/library/windows/hardware/dn940129%28v=vs.85%29.aspx
blogs.technet.com/b/mniehaus/archive/2015/09/16/windows-10-reducing-the-disk-footprint.aspx