An MS account is needed for some functions like installing an app from the Store. The account can be specified 'per app' without changing from a local account for Windows login. The app remembers it so no need to keep entering it.
I always set up computers for my clients with a local account. The only problem is that when an app (e.g. Store) requests an account and the user specifies/creates one, the following screen has a deliberately vaguely worded request for permission to switch to MS account for the computer login.
Although this is optional, many users just agree because over the past decades the wise choice was, if unsure, to accept Microsoft recommendations. This has become a collective habit, even some techs on these forums believe a MS account computer login is needed for all functionality.
Since Windows 8 though, Microsoft recommendations are not always in the best interest of ordinary mum & dad users (or any users, some would argue). Many don't even read the screen requesting the switch to MS account login, so the next time they turn on their computer (e.g. the next day) they suddenly have an MS account login. It "magically appeared on the computer overnight"