There are a couple of benefits to using your online Microsoft account (Outlook or Live) to sign into Windows 10. On Windows Weekly episode 506 on the TWiT network, Paul Thurrott talked about these benefits, and you can find his article on the subject at thurrott.com.
The first benefit to signing into Windows 10 with a Microsoft account is that it will sync all of your preferences across all of your PCs. This means your desktop wallpaper, language preferences, color schemes, and more will all be the same on all of your computers running Windows 10.
The second reason to do this is for convenience. If you use your Microsoft account to sign into Windows 10, that account will be applied to all of the apps that come bundled with Windows. This means that when you first run any of the Windows apps, including Groove Music, One Drive, and the Edge browser, you won't have to sign in separately.
You can still sign into Windows 10 locally, but you won't get the benefits mentioned here. To find out more about whether or not to use a Microsoft account to sign into Windows, check out this article at thurrott.com.