Is there a particular reason why we are able to use the metal map for both the specular and the metallic surface shader?

Technically they are different maps altogether because they both use different channels depending on how they are used. So I wouldn’t use them interchangeably unless you needed to because it won’t look 100% correct. Most of the time though these maps tend to look identical in terms of what parts should be shinier and darker so that’s why I decided to use it in this case.