Windows 8.1 is an improved version of Windows 8, the start of modern desktop operating system by Microsoft.
Windows 8 was released after Windows 7 and before Windows 10. Microsoft almost overhauled the operating system to make it a modern desktop OS. Windows 8 had some serious flaws which were badly criticized by public. Therefore Microsoft released Windows 8.1 to fix those issues and introduced some new features.
Windows 8/8.1 was expected to become the best desktop OS, but it was a flop! Actually. Despite of years of efforts, Windows 8/8.1 could not prove itself a usable OS (while Microsoft was dreaming it would rock).
Windows 8.1 has its own features. Some users prefer Windows 8.1 to Windows 10 and Windows 7. That makes sense. Windows 8.1 has its own advantages and features that are not found in any other operating system.
Windows 8.1 Features
Boot direct to desktop
In Windows 8, the start screen used as first screen after user login. The users had to click desktop tile from start screen. That was kind of painful extra step. It got fixed in 8.1 where traditional desktop is the first screen after login.
Set default apps
Do you like to keep old Internet Explorer browser as your default Internet browser? Nobody likes it (even Microsoft employees, unless forced, probably). Windows 8.1 allows switching default browser and other apps like photo viewer, media player etc.
No, it is not that you are thinking. It is the same Windows 8 start button but with some improvements like right click options and a few more enhancements. Yet it does not bring start menu like Windows 7 or Windows 10 etc.
If you ask us, start button and start menu contributed above 70% towards Windows 8/8.1 failure. Microsoft stuck with it unless total destruction of Windows 8 usage.
Tiles could be resized to small, medium. large and wide sizes. Though not amazing, it was useful feature to organize start screen.
Auto app update
Windows metro apps can be set up to update automatically to newer version. You can enable/disable Automatically update apps option.