I am curious to know why we always need to register our CRM online instance on an Azure Active Directory in order to authenticate the Web API while accessing from outside CRM domain.
That is, for example, if I need to access CRM online instance through another website using CRM's Web API endpoint, then I must register my CRM instance to Azure Active Directory. Though I am aware that, its a very nominal charge to create an Azure Active Directory, still I would need to subscribe to Azure even if I just want to perform some general research for CRM connectivity through Web API.
Why this is must? Are there any security considerations behind this?
Why can't we use the same authentication mechanism as we used to do with Organization service?
Any details on this will be much appreciated.
The CRM WebAPI uses OAuth2 and Azure AD is the only currently supported authentication platform to provide this (Windows Server 2016 will support OAuth2 for on-premise).
The Organisation service is a WCF service and as such uses SOAP for authentication and authorization. This is an entirely different technology stack that brings it's own set of problems, many of which the OAuth2 protocol tries to solve in this scenario.
Although you manage your CRM Online users through the Office 365 portal the underlying technology for these accounts is also Azure AD. Check if you can use this existing AD tenant created as part of your subscription rather than having to create another.