I hope someone could provide an explanation and specific scenarios about why someone should put Azure Front Door in front of Azure API Management.
Well, for one Azure Front Door includes Web Application Firewall so from a security perspective it makes sense. It also offers DDos protection.
Also, if you later on need additional services like web apps, functions or whetever it makes sense to use Front Door as the single point of entry, and disallow direct access to the resources it protects.
There is a bunch of other features that may not provided by APIM, see the docs to see if there is anything that benefits your scenario if not for security reasons.