Azure provides the preview view for basic evaluation purposes. It is not specifically for previews of new services. Previews require separate service terms and are made available to you on the condition that you agree to these terms of use required for accessing the Azure services.
You can use the new services using portal.azure.com and preview.portal.azure.com normally.
We can say that preview is no different from the Azure portal as far as basic services are involved. It only differs with a change in versions.
To learn more about services provided by Azure, check this video out:
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