I don't understand most of the scenarios for using Azure AD

According to the docs, azure ad join is good for the following scenarios:

1. Businesses largely in the cloud

2. schools

3. Retail

I get the first one. If users will only need to access cloud services, then Azure AD join seems like a good fit. But how do the others actually work? If a student only has an Azure AD account, how would they access non-cloud services? Same for retail. It suggests giving long term employees AD accounts and short term employees Azure AD accounts. I get that Azure AD accounts could be easier to manage, but how would those employees get access to resources other than Office 365 and Office, such as a local network share or printer?

edit: to clarify, I'm talking about Azure AD Join. I get the purpose of Azure AD in general, just not this piece.