On Demand Audit is not required to audit Azure AD if you have Change Auditor. It however does have some potential benefits over CA, depending on your use case.
On Demand Audit collects the same audit data from Azure AD as CA does (when an Azure audit template is deployed), however it does not store the data in an on-prem database, but rather in Azure. There is also no on-prem application required. This helps to lower your management overhead.
The interface in On Demand Audit can be considered easier to use, especially for end users who do not necessarily need to see, or use, all of the options found in the CA Client. In addition, you can provide more granular rights in On Demand Audit to be able to have more control over what users can do. With ODA being a SaaS application, users can access it more easily as there is no need to install a client or remote into another machine.
ODA also allows you to send your CA data to ODA. Currently ODA only audits Azure AD, however when you integrate ODA with your on-prem CA Installation, CA forwards all on-prem AD Audit data to ODA. This allows you to provide a single, in the cloud, interface for end users to search across all Azure AD and on-prem AD data.
There is a free 30 day unrestricted trial that will allow you to setup and configure ODA (as well as the integration with CA) and test it out to see if it meets your needs.
If you require more information about Change Auditor or On Demand Audit, please contact Sales.