Why is Azure AD needed when deploying a VM with on-prem AD and on-site VPN?

From @CloudFronts via Twitter:

Hi! We are trying to deploy AX 2012 R3 on Azure and had a question: Why do we need an Azure AD when we have an On-Prem AD & a Vnet on Azure with a site-to-site VPN?

I am trying to understand why your AX deployment steps require an Azure AD for my AX when I already have an on-premise network with a site-to-site VPN. Your document -https://technet.microsoft.com/en-us/library/dn948285.aspx …

Thanks,

@AzureSupport