What am I actually paying for when I buy the Windows 10 key?

I have the free version that you can download from Microsoft where it says to activate a product key in the bottom right corner but I'm a little put off from doing that because it doesn't say exactly what I'm getting with the $110 it wants me to pay for it. In fact the only thing I could find so far that required the key was being able to customize the theme and background.

Is there something else I get out of it?